Image encoding/decoding method and apparatus, and recording medium storing bitstream

ABSTRACT

The present specification discloses a method of decoding an image. The method of decoding an image according to the method includes: obtaining transform skip information of a current block from a bitstream; obtaining multiple transform selection information of the current block on the basis of the transform skip information from the bitstream; and performing inverse transform on the current block on the basis of the multiple transform selection information, wherein the multiple transform selection information is used to set each of a horizontal transform type and a vertical transform type.

TECHNICAL FIELD

The present invention relates to a method and apparatus forencoding/decoding an image, more particularly, the present inventionrelates to a method and apparatus for encoding/decoding using selectivetransform.

BACKGROUND ART

Recently, the demand for high quality images, such as ultra highdefinition (UHD) images that can provide high resolution, wider colorspace, and excellent image quality is increasing in variousapplications.

As image data becomes higher resolution and higher quality, the amountof data thereof is relatively increased compared to that of the existingimage data. Accordingly, when image data is transmitted usingcommunication media such as wired/wireless broadband or variousbroadcasting medium such as satellite, terrestrial, IP network,wireless, cable, and mobile communication networks, or stored usingvarious storage media such as CD, DVD, USB, and HD-DVD, the transmissioncost and the storage cost increase.

In order to solve these problems, which are inevitably intensified asimage data becomes high resolution and high quality, and to serviceimages having higher resolution and image quality, an imageencoding/decoding technique of high efficiency is required.

An image compression technology that has been developed or is beingdeveloped for this purpose, there are various techniques, such as aninter prediction technique for predicting pixel values included in acurrent picture from a picture before or after the current picture, anintra prediction technique for predicting pixel values included in thecurrent picture using pixel information in the current picture, atransform and quantization technique for compressing energy of aresidual signal remaining as a prediction error, and an entropy codingand arithmetic coding technique for assigning short codes tohigh-frequency values and long codes to low-frequency values, in whichimage data can be effectively compressed and transmitted or stored usingsuch image compression techniques.

DISCLOSURE Technical Problem

An objective of the present invention is to provide a method andapparatus for determining transform skip for each of the horizontal andvertical directions of a transform block.

An objective of the present invention is to provide a method andapparatus for encoding transform skip information determined for each ofhorizontal and vertical directions of a transform block.

An objective of the present invention is to provide a method andapparatus for decoding transform skip information encoded for each ofhorizontal and vertical directions of a transform block.

An objective of the present invention is to provide a method andapparatus for decoding compressed image information by performing orskipping transform independently for the horizontal and verticaldirections according to the transform skip information for each of thedecoded horizontal and vertical directions.

An objective of the present invention is to provide a method andapparatus for applying the above methods and apparatus independently oridentically for each channel of an image.

It is an objective of the present invention to provide a method and anapparatus for selecting multiple transform or determining whether toskip or perform transform independently for each of the horizontal andvertical directions of a transform block.

An object of the present invention is to provide a signaling order oftransform information for a block to be decoded for efficienttransmission of a transform method.

An object of the present invention is to provide an encoding method andapparatus for performing the processes of transform skipping andmultiple transform selection in each of the horizontal and verticaldirections using whether to skip transform on all channels as acandidate for multiple transform selection.

An object of the present invention is to provide a method and anapparatus for transmitting the determination simultaneously with othertransforms, skipping a flag indicating whether the transform is skippedand using the transform skipping as a candidate for multiple transformselection.

Technical Solution

A method of decoding an image according to an embodiment of the presentinvention, the method may comprise obtaining transform skip informationof a current block from a bitstream; obtaining multiple transformselection information of the current block on the basis of the transformskip information from the bitstream; and performing inverse transform onthe current block on the basis of the multiple transform selectioninformation, wherein the multiple transform selection information isused to set each of a horizontal transform type and a vertical transformtype.

In the method of decoding an image according to the present invention,wherein the multiple transform selection information indicates whetherthe horizontal transform type and the vertical transform type are thesame.

In the method of decoding an image according to the present invention,wherein the multiple transform selection information is indexinformation indicating a transform type set applied to the horizontaltransform type and the vertical transform type.

In the method of decoding an image according to the present invention,wherein the acquiring of the transform skip information of the currentblock includes: obtaining transform skip information of the currentblock when a horizontal size and a vertical size of the current blockare less than or equal to a predetermined size.

In the method of decoding an image according to the present invention,wherein the acquiring of the transform skip information of the currentblock includes: obtaining maximum transform skip size information fromthe bitstream; and obtaining transform skip information of the currentblock when the horizontal size and the vertical size of the currentblock are less than or equal to the maximum transform skip size.

In the method of decoding an image according to the present invention,wherein the transform skip information includes horizontal transformskip information and vertical transform skip information.

A method of decoding an image according to an embodiment of the presentinvention, the method may comprise obtaining transform selectioninformation of a current block from a bitstream; determining whether toperform inverse transform on the current block and a transform type onthe basis of the transform selection information; and performing inversetransform on the current block according to the determination.

In the method of decoding an image according to the present invention,wherein the inverse transform is secondary inverse transform performedbetween dequantization and primary inverse transform.

A method of encoding an image according to an embodiment of the presentinvention, the method may comprise determining whether to performtransform skip of the current block; when the transform skip is notperformed on the current block, determining a horizontal transform typeand a vertical transform type of the current block; performing thetransform on the current block on the basis of the horizontal transformtype and the vertical transform type; and encoding transform skipinformation indicating whether to perform the transform skip of thecurrent block and multiple transform selection information indicatingthe horizontal transform type and the vertical transform type of thecurrent block.

In the method of encoding an image according to the present invention,wherein the multiple transform selection information indicates whetherthe horizontal transform type and the vertical transform type are thesame.

In the method of encoding an image according to the present invention,wherein the multiple transform selection information is indexinformation indicating a transform type set applied to the horizontaltransform type and the vertical transform type.

In the method of encoding an image according to the present invention,wherein the transform skip information of the current block is notencoded when the horizontal size and the vertical size of the currentblock are less than or equal to a predetermined size.

In the method of encoding an image according to the present invention,wherein the transform skip information includes horizontal transformskip information and vertical transform skip information.

A method of encoding an image according to an embodiment of the presentinvention, the method may comprise determining whether to performtransform on a current block and a transform type; performing thetransform on the current block according to the determination; andencoding transform selection information indicating whether thetransform is performed on the current block and a transform type.

In the method of encoding an image according to the present invention,wherein the transform is secondary transform performed between primarytransform and quantization.

A non-transitory computer readable recording medium storing a bitstreamdecoded by an image decoding apparatus to an embodiment of the presentinvention, wherein the bitstream includes transform skip information ofa current block and multiple transform selection information of thecurrent block; the transform skip information indicates whether inversetransform is performed on the current block; the multiple transformselection information indicates a horizontal transform type and avertical transform type applied to inverse transform of the currentblock; and the image decoding apparatus obtains the multiple transformselection information on the basis of the transform skip information.

Advantageous Effects

According to the present invention, by independently performingtransform for each of the horizontal and vertical directions of a block,there is an effect that not only the encoding efficiency but also theimage quality can be improved.

The present invention has the effect of improving the encodingefficiency and the image quality without additional signaling becausethe information for determining whether to perform the transform issignaled in combination with the multiple transform selectioninformation.

According to the present invention, even when the change in the spatialpixel value is very large or very sharp in the corresponding block ofthe image to be compressed so that the energy of the image is notconcentrated at low frequency even though transform is performed, andthe low frequency component is mainly maintained and the high frequencycomponent is removed or the quantization is strongly applied, in thecompression process, thereby causing serious reduction of the imagequality, there is an effect that the encoder can efficiently instruct toskip performing block transform for each of horizontal and verticaldirections without a large overhead burden, thereby increasing the imagecompression rate and reducing the deterioration of image quality.

According to the present invention, an efficient transform method can beselected for each of the horizontal and vertical directions of eachblock in combination of the multiple transform selection process and thetransform skip process.

According to the invention, a combination of transform skip and multipletransform can be used without additional flags by skipping signaling ofthe transform skip flag.

According to the present invention, it is possible to improve encodingand decoding efficiency of an image.

According to the present invention, it is possible to reduce thecomputational complexity of the encoder and the decoder of an image.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of an encodingapparatus according to an embodiment of the present invention

FIG. 2 is a block diagram illustrating a configuration of a decodingapparatus according to an embodiment of the present invention.

FIG. 3 is a diagram schematically illustrating a division structure ofan image when encoding and decoding the image.

FIG. 4 is a diagram illustrating an embodiment of an intra predictionprocess.

FIG. 5 is a diagram illustrating an embodiment of an inter predictionprocess.

FIG. 6 is a diagram illustrating a process of transform andquantization.

FIG. 7 is a diagram illustrating reference samples available for intraprediction.

FIGS. 8 and 9 are diagrams illustrating an embodiment of a multipletransform technique.

FIGS. 10 and 11 are diagrams illustrating an embodiment of transformskip information.

FIGS. 12 and 13 are diagrams illustrating an embodiment in which atransform skip mode is represented by an identity matrix (IDT) withoutsignaling transform skip information.

FIGS. 14 and 15 are diagrams illustrating various embodiments in whichinformation indicating whether each of the horizontal transform and thevertical transform use the same transform kernel is used.

FIG. 16 is a diagram illustrating an embodiment of using bothinformation (mts_cu_flag) indicating whether to use multiple transformsand information (same_transform_hor_ver_flag) indicating whether thehorizontal transform and the vertical transform use the same transformkernel.

FIGS. 17 and 18 are diagrams illustrating an embodiment in whichtransform skip information is signaled after signaling informationindicating whether to use multiple transforms.

FIG. 19 is a diagram illustrating an embodiment in which informationindicating whether to use multiple transforms is signaled aftersignaling transform skip information.

FIG. 20 is a diagram illustrating an embodiment in which signaling ofmultiple transform usage information is determined on the basis ofhorizontal transform skip information and vertical transform skipinformation.

FIG. 21 is a flowchart illustrating an image decoding method accordingto an embodiment of the present invention.

FIG. 22 is a flowchart illustrating an image decoding method accordingto an embodiment of the present invention.

FIG. 23 is a flowchart illustrating an image encoding method accordingto an embodiment of the present invention.

FIG. 24 is a flowchart illustrating an image encoding method accordingto an embodiment of the present invention.

MODE FOR INVENTION

A variety of modifications may be made to the present invention andthere are various embodiments of the present invention, examples ofwhich will now be provided with reference to drawings and described indetail. However, the present invention is not limited thereto, althoughthe exemplary embodiments can be construed as including allmodifications, equivalents, or substitutes in a technical concept and atechnical scope of the present invention. The similar reference numeralsrefer to the same or similar functions in various aspects. In thedrawings, the shapes and dimensions of elements may be exaggerated forclarity. In the following detailed description of the present invention,references are made to the accompanying drawings that show, by way ofillustration, specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to implement the present disclosure.Various embodiments of the present disclosure, although different, arenot necessarily mutually exclusive. For example, specific features,structures, and characteristics described herein, in connection with oneembodiment, may be implemented within other embodiments withoutdeparting from the spirit and scope of the present disclosure. Inaddition, it should be understood that the location or arrangement ofindividual elements within each disclosed embodiment may be modifiedwithout departing from the spirit and scope of the present disclosure.The following detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present disclosure is defined onlyby the appended claims, appropriately interpreted, along with the fullrange of equivalents to what the claims claim.

Terms used in the specification, ‘first’, ‘second’, etc. can be used todescribe various components, but the components are not to be construedas being limited to the terms. The terms are only used to differentiateone component from other components. For example, the ‘first’ componentmay be named the ‘second’ component without departing from the scope ofthe present invention, and the ‘second’ component may also be similarlynamed the ‘first’ component. The term ‘and/or’ includes a combination ofa plurality of items or any one of a plurality of terms.

It will be understood that when an element is simply referred to asbeing ‘connected to’ or ‘coupled to’ another element without being‘directly connected to’ or ‘directly coupled to’ another element in thepresent description, it may be ‘directly connected to’ or ‘directlycoupled to’ another element or be connected to or coupled to anotherelement, having the other element intervening therebetween. In contrast,it should be understood that when an element is referred to as being“directly coupled” or “directly connected” to another element, there areno intervening elements present.

Furthermore, constitutional parts shown in the embodiments of thepresent invention are independently shown so as to representcharacteristic functions different from each other. Thus, it does notmean that each constitutional part is constituted in a constitutionalunit of separated hardware or software. In other words, eachconstitutional part includes each of enumerated constitutional parts forconvenience. Thus, at least two constitutional parts of eachconstitutional part may be combined to form one constitutional part orone constitutional part may be divided into a plurality ofconstitutional parts to perform each function. The embodiment where eachconstitutional part is combined and the embodiment where oneconstitutional part is divided are also included in the scope of thepresent invention, if not departing from the essence of the presentinvention.

The terms used in the present specification are merely used to describeparticular embodiments, and are not intended to limit the presentinvention. An expression used in the singular encompasses the expressionof the plural, unless it has a clearly different meaning in the context.In the present specification, it is to be understood that terms such as“including”, “having”, etc. are intended to indicate the existence ofthe features, numbers, steps, actions, elements, parts, or combinationsthereof disclosed in the specification, and are not intended to precludethe possibility that one or more other features, numbers, steps,actions, elements, parts, or combinations thereof may exist or may beadded. In other words, when a specific element is referred to as being“included”, elements other than the corresponding element are notexcluded, but additional elements may be included in embodiments of thepresent invention or the scope of the present invention.

In addition, some of constituents may not be indispensable constituentsperforming essential functions of the present invention but be selectiveconstituents improving only performance thereof. The present inventionmay be implemented by including only the indispensable constitutionalparts for implementing the essence of the present invention except theconstituents used in improving performance. The structure including onlythe indispensable constituents except the selective constituents used inimproving only performance is also included in the scope of the presentinvention.

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. In describingexemplary embodiments of the present invention, well-known functions orconstructions will not be described in detail since they mayunnecessarily obscure the understanding of the present invention. Thesame constituent elements in the drawings are denoted by the samereference numerals, and a repeated description of the same elements willbe skipped.

Hereinafter, an image may mean a picture configuring a video, or maymean the video itself. For example, “encoding or decoding or both of animage” may mean “encoding or decoding or both of a moving picture” andmay mean “encoding or decoding or both of one image among images of amoving picture.”

Hereinafter, terms “moving picture” and “video” may be used as the samemeaning and be replaced with each other.

Hereinafter, a target image may be an encoding target image which is atarget of encoding and/or a decoding target image which is a target ofdecoding. Also, a target image may be an input image inputted to anencoding apparatus, and an input image inputted to a decoding apparatus.Here, a target image may have the same meaning with the current image.

Hereinafter, terms “image”, “picture, “frame” and “screen” may be usedas the same meaning and be replaced with each other.

Hereinafter, a target block may be an encoding target block which is atarget of encoding and/or a decoding target block which is a target ofdecoding. Also, a target block may be the current block which is atarget of current encoding and/or decoding. For example, terms “targetblock” and “current block” may be used as the same meaning and bereplaced with each other.

Hereinafter, terms “block” and “unit” may be used as the same meaningand be replaced with each other. Or a “block” may represent a specificunit.

Hereinafter, terms “region” and “segment” may be replaced with eachother.

Hereinafter, a specific signal may be a signal representing a specificblock. For example, an original signal may be a signal representing atarget block. A prediction signal may be a signal representing aprediction block. A residual signal may be a signal representing aresidual block.

In embodiments, each of specific information, data, flag, index, elementand attribute, etc. may have a value. A value of information, data,flag, index, element and attribute equal to “0” may represent a logicalfalse or the first predefined value. In other words, a value “0”, afalse, a logical false and the first predefined value may be replacedwith each other. A value of information, data, flag, index, element andattribute equal to “1” may represent a logical true or the secondpredefined value. In other words, a value “1”, a true, a logical trueand the second predefined value may be replaced with each other.

When a variable i or j is used for representing a column, a row or anindex, a value of i may be an integer equal to or greater than 0, orequal to or greater than 1. That is, the column, the row, the index,etc. may be counted from 0 or may be counted from 1.

Description of Terms

Encoder: means an apparatus performing encoding. That is, means anencoding apparatus.

Decoder: means an apparatus performing decoding. That is, means adecoding apparatus.

Block: is an M×N array of a sample. Herein, M and N may mean positiveintegers, and the block may mean a sample array of a two-dimensionalform. The block may refer to a unit. A current block my mean an encodingtarget block that becomes a target when encoding, or a decoding targetblock that becomes a target when decoding. In addition, the currentblock may be at least one of an encode block, a prediction block, aresidual block, and a transform block.

Sample: is a basic unit constituting a block. It may be expressed as avalue from 0 to 2Bd−1 according to a bit depth (Bd). In the presentinvention, the sample may be used as a meaning of a pixel. That is, asample, a pel, a pixel may have the same meaning with each other.

Unit: may refer to an encoding and decoding unit. When encoding anddecoding an image, the unit may be a region generated by partitioning asingle image. In addition, the unit may mean a subdivided unit when asingle image is partitioned into subdivided units during encoding ordecoding. That is, an image may be partitioned into a plurality ofunits. When encoding and decoding an image, a predetermined process foreach unit may be performed. A single unit may be partitioned intosub-units that have sizes smaller than the size of the unit. Dependingon functions, the unit may mean a block, a macroblock, a coding treeunit, a code tree block, a coding unit, a coding block), a predictionunit, a prediction block, a residual unit), a residual block, atransform unit, a transform block, etc. In addition, in order todistinguish a unit from a block, the unit may include a luma componentblock, a chroma component block associated with the luma componentblock, and a syntax element of each color component block. The unit mayhave various sizes and forms, and particularly, the form of the unit maybe a two-dimensional geometrical figure such as a square shape, arectangular shape, a trapezoid shape, a triangular shape, a pentagonalshape, etc. In addition, unit information may include at least one of aunit type indicating the coding unit, the prediction unit, the transformunit, etc., and a unit size, a unit depth, a sequence of encoding anddecoding of a unit, etc.

Coding Tree Unit: is configured with a single coding tree block of aluma component Y, and two coding tree blocks related to chromacomponents Cb and Cr. In addition, it may mean that including the blocksand a syntax element of each block. Each coding tree unit may bepartitioned by using at least one of a quad-tree partitioning method, abinary-tree partitioning method and ternary-tree partitioning method toconfigure a lower unit such as coding unit, prediction unit, transformunit, etc. It may be used as a term for designating a sample block thatbecomes a process unit when encoding/decoding an image as an inputimage. Here, the quad-tree may mean a quarternary-tree.

When the size of the coding block is within a predetermined range, thedivision is possible using only quad-tree partitioning. Here, thepredetermined range may be defined as at least one of a maximum size anda minimum size of a coding block in which the division is possible usingonly quad-tree partitioning. Information indicating a maximum/minimumsize of a coding block in which quad-tree partitioning is allowed may besignaled through a bitstream, and the information may be signaled in atleast one unit of a sequence, a picture parameter, a tile group, or aslice (segment). Alternatively, the maximum/minimum size of the codingblock may be a fixed size predetermined in the coder/decoder. Forexample, when the size of the coding block corresponds to 256×256 to64×64, the division is possible only using quad-tree partitioning.Alternatively, when the size of the coding block is larger than the sizeof the maximum conversion block, the division is possible only usingquad-tree partitioning. Herein, the block to be divided may be at leastone of coding blocks and a transform block. In this case, informationindicating the division of the coded block (for example, split_flag) maybe a flag indicating whether or not to perform the quad-treepartitioning. When the size of the coding block falls within apredetermined range, the division is possible only using binary tree orternary tree partitioning. In this case, the above description of thequad-tree partitioning may be applied to binary tree partitioning orternary tree partitioning in the same manner.

Coding Tree Block: may be used as a term for designating any one of a Ycoding tree block, Cb coding tree block, and Cr coding tree block.

Neighbor Block: may mean a block adjacent to a current block. The blockadjacent to the current block may mean a block that comes into contactwith a boundary of the current block, or a block positioned within apredetermined distance from the current block. The neighbor block maymean a block adjacent to a vertex of the current block. Herein, theblock adjacent to the vertex of the current block may mean a blockvertically adjacent to a neighbor block that is horizontally adjacent tothe current block, or a block horizontally adjacent to a neighbor blockthat is vertically adjacent to the current block.

Reconstructed Neighbor block: may mean a neighbor block adjacent to acurrent block and which has been already spatially/temporally encoded ordecoded. Herein, the reconstructed neighbor block may mean areconstructed neighbor unit. A reconstructed spatial neighbor block maybe a block within a current picture and which has been alreadyreconstructed through encoding or decoding or both. A reconstructedtemporal neighbor block is a block at a corresponding position as thecurrent block of the current picture within a reference image, or aneighbor block thereof.

Unit Depth: may mean a partitioned degree of a unit. In a treestructure, the highest node(Root Node) may correspond to the first unitwhich is not partitioned. Also, the highest node may have the leastdepth value. In this case, the highest node may have a depth of level 0.A node having a depth of level 1 may represent a unit generated bypartitioning once the first unit. A node having a depth of level 2 mayrepresent a unit generated by partitioning twice the first unit. A nodehaving a depth of level n may represent a unit generated by partitioningn-times the first unit. A Leaf Node may be the lowest node and a nodewhich cannot be partitioned further. A depth of a Leaf Node may be themaximum level. For example, a predefined value of the maximum level maybe 3. A depth of a root node may be the lowest and a depth of a leafnode may be the deepest. In addition, when a unit is expressed as a treestructure, a level in which a unit is present may mean a unit depth.

Bitstream: may mean a bitstream including encoding image information.

Parameter Set: corresponds to header information among a configurationwithin a bitstream. At least one of a video parameter set, a sequenceparameter set, a picture parameter set, and an adaptation parameter setmay be included in a parameter set. In addition, a parameter set mayinclude a slice header, a tile group header, and tile headerinformation. The term “tile group” means a group of tiles and has thesame meaning as a slice.

An adaptation parameter set may mean a parameter set that can be sharedby being referred to in different pictures, subpictures, slices, tilegroups, tiles, or bricks. In addition, information in an adaptationparameter set may be used by referring to different adaptation parametersets for a subpicture, a slice, a tile group, a tile, or a brick insidea picture.

In addition, regarding the adaptation parameter set, differentadaptation parameter sets may be referred to by using identifiers ofdifferent adaptation parameter sets for a subpicture, a slice, a tilegroup, a tile, or a brick inside a picture.

In addition, regarding the adaptation parameter set, differentadaptation parameter sets may be referred to by using identifiers ofdifferent adaptation parameter sets for a slice, a tile group, a tile,or a brick inside a subpicture.

In addition, regarding the adaptation parameter set, differentadaptation parameter sets may be referred to by using identifiers ofdifferent adaptation parameter sets for a tile or a brick inside aslice.

In addition, regarding the adaptation parameter set, differentadaptation parameter sets may be referred to by using identifiers ofdifferent adaptation parameter sets for a brick inside a tile.

Information on an adaptation parameter set identifier may be included ina parameter set or a header of the subpicture, and an adaptationparameter set corresponding to the adaptation parameter set identifiermay be used for the subpicture.

The information on the adaptation parameter set identifier may beincluded in a parameter set or a header of the tile, and an adaptationparameter set corresponding to the adaptation parameter set identifiermay be used for the tile.

The information on the adaptation parameter set identifier may beincluded in a header of the brick, and an adaptation parameter setcorresponding to the adaptation parameter set identifier may be used forthe brick.

The picture may be partitioned into one or more tile rows and one ormore tile columns.

The subpicture may be partitioned into one or more tile rows and one ormore tile columns within a picture. The subpicture may be a regionhaving the form of a rectangle/square within a picture and may includeone or more CTUs. In addition, at least one or more tiles/bricks/slicesmay be included within one subpicture.

The tile may be a region having the form of a rectangle/square within apicture and may include one or more CTUs. In addition, the tile may bepartitioned into one or more bricks.

The brick may mean one or more CTU rows within a tile. The tile may bepartitioned into one or more bricks, and each brick may have at leastone or more CTU rows. A tile that is not partitioned into two or moremay mean a brick.

The slice may include one or more tiles within a picture and may includeone or more bricks within a tile.

Parsing: may mean determination of a value of a syntax element byperforming entropy decoding or may mean the entropy decoding itself.

Symbol: may mean at least one of a syntax element, a coding parameter,and a transform coefficient value of an encoding/decoding target unit.In addition, the symbol may mean an entropy encoding target or anentropy decoding result.

Prediction Mode: may be information indicating a mode encoded/decodedwith intra prediction or a mode encoded/decoded with inter prediction.

Prediction Unit: may mean a basic unit when performing prediction suchas inter-prediction, intra-prediction, inter-compensation,intra-compensation, and motion compensation. A single prediction unitmay be partitioned into a plurality of partitions having a smaller sizeor may be partitioned into a plurality of lower prediction units. Aplurality of partitions may be a basic unit in performing prediction orcompensation. A partition which is generated by dividing a predictionunit may also be a prediction unit.

Prediction Unit Partition: may mean a form obtained by partitioning aprediction unit.

Reference picture list: may refer to a list including one or morereference pictures used for inter prediction or motion compensation.There are several types of usable reference picture lists, including LC(List combined), L0 (List 0), L1 (List 1), L2 (List 2), L3 (List 3).

Inter prediction indicator: may refer to a direction of inter prediction(unidirectional prediction, bidirectional prediction, etc.) of a currentblock. Alternatively, it may refer to the number of reference picturesused to generate a prediction block of a current block. Alternatively,it may refer to the number of prediction blocks used at the time ofperforming inter prediction or motion compensation on a current block.

Prediction list utilization flag: indicates whether a prediction blockis generated using at least one reference picture in a specificreference picture list. An inter prediction indicator can be derivedusing a prediction list utilization flag, and conversely, a predictionlist utilization flag can be derived using an inter predictionindicator. For example, when the prediction list utilization flag has afirst value of zero (0), it means that a reference picture in areference picture list is not used to generate a prediction block. Onthe other hand, when the prediction list utilization flag has a secondvalue of one (1), it means that a reference picture list is used togenerate a prediction block.

Reference picture index: may refer to an index indicating a specificreference picture in a reference picture list.

Reference picture: may mean a reference picture which is referred to bya specific block for the purposes of inter prediction or motioncompensation of the specific block. Alternatively, the reference picturemay be a picture including a reference block referred to by a currentblock for inter prediction or motion compensation. Hereinafter, theterms “reference picture” and “reference image” have the same meaningand can be interchangeably.

Motion vector: may be a two-dimensional vector used for inter predictionor motion compensation. The motion vector may mean an offset between anencoding/decoding target block and a reference block. For example, (mvX,mvY) may represent a motion vector. Here, mvX may represent a horizontalcomponent and mvY may represent a vertical component.

Search range: may be a two-dimensional region which is searched toretrieve a motion vector during inter prediction. For example, the sizeof the search range may be M×N. Here, M and N are both integers.

Motion vector candidate: may refer to a prediction candidate block or amotion vector of the prediction candidate block when predicting a motionvector. In addition, a motion vector candidate may be included in amotion vector candidate list.

Motion vector candidate list: may mean a list composed of one or moremotion vector candidates.

Motion vector candidate index: may mean an indicator indicating a motionvector candidate in a motion vector candidate list. Alternatively, itmay be an index of a motion vector predictor.

Motion information: may mean information including at least one of theitems including a motion vector, a reference picture index, an interprediction indicator, a prediction list utilization flag, referencepicture list information, a reference picture, a motion vectorcandidate, a motion vector candidate index, a merge candidate, and amerge index.

Merge candidate list: may mean a list composed of one or more mergecandidates.

Merge candidate: may mean a spatial merge candidate, a temporal mergecandidate, a combined merge candidate, a combined bi-predictive mergecandidate, or a zero-merge candidate. The merge candidate may includemotion information such as an inter prediction indicator, a referencepicture index for each list, a motion vector, a prediction listutilization flag, and an inter prediction indicator.

Merge index: may mean an indicator indicating a merge candidate in amerge candidate list. Alternatively, the merge index may indicate ablock from which a merge candidate has been derived, among reconstructedblocks spatially/temporally adjacent to a current block. Alternatively,the merge index may indicate at least one motion information of a mergecandidate.

Transform Unit: may mean a basic unit when performing encoding/decodingsuch as transform, inverse-transform, quantization, dequantization,transform coefficient encoding/decoding of a residual signal. A singletransform unit may be partitioned into a plurality of lower-leveltransform units having a smaller size. Here,transformation/inverse-transformation may comprise at least one amongthe first transformation/the first inverse-transformation and the secondtransformation/the second inverse-transformation.

Scaling: may mean a process of multiplying a quantized level by afactor. A transform coefficient may be generated by scaling a quantizedlevel. The scaling also may be referred to as dequantization.

Quantization Parameter: may mean a value used when generating aquantized level using a transform coefficient during quantization. Thequantization parameter also may mean a value used when generating atransform coefficient by scaling a quantized level duringdequantization. The quantization parameter may be a value mapped on aquantization step size.

Delta Quantization Parameter: may mean a difference value between apredicted quantization parameter and a quantization parameter of anencoding/decoding target unit.

Scan: may mean a method of sequencing coefficients within a unit, ablock or a matrix. For example, changing a two-dimensional matrix ofcoefficients into a one-dimensional matrix may be referred to asscanning, and changing a one-dimensional matrix of coefficients into atwo-dimensional matrix may be referred to as scanning or inversescanning.

Transform Coefficient: may mean a coefficient value generated aftertransform is performed in an encoder. It may mean a coefficient valuegenerated after at least one of entropy decoding and dequantization isperformed in a decoder. A quantized level obtained by quantizing atransform coefficient or a residual signal, or a quantized transformcoefficient level also may fall within the meaning of the transformcoefficient.

Quantized Level: may mean a value generated by quantizing a transformcoefficient or a residual signal in an encoder. Alternatively, thequantized level may mean a value that is a dequantization target toundergo dequantization in a decoder. Similarly, a quantized transformcoefficient level that is a result of transform and quantization alsomay fall within the meaning of the quantized level.

Non-zero Transform Coefficient: may mean a transform coefficient havinga value other than zero, or a transform coefficient level or a quantizedlevel having a value other than zero.

Quantization Matrix: may mean a matrix used in a quantization process ora dequantization process performed to improve subjective or objectiveimage quality. The quantization matrix also may be referred to as ascaling list.

Quantization Matrix Coefficient: may mean each element within aquantization matrix. The quantization matrix coefficient also may bereferred to as a matrix coefficient.

Default Matrix: may mean a predetermined quantization matrixpreliminarily defined in an encoder or a decoder.

Non-default Matrix: may mean a quantization matrix that is notpreliminarily defined in an encoder or a decoder but is signaled by auser.

Statistic Value: a statistic value for at least one among a variable, anencoding parameter, a constant value, etc. which have a computablespecific value may be one or more among an average value, a sum value, aweighted average value, a weighted sum value, the minimum value, themaximum value, the most frequent value, a median value, an interpolatedvalue of the corresponding specific values.

FIG. 1 is a block diagram showing a configuration of an encodingapparatus according to an embodiment to which the present invention isapplied.

An encoding apparatus 100 may be an encoder, a video encoding apparatus,or an image encoding apparatus. A video may include at least one image.The encoding apparatus 100 may sequentially encode at least one image.

Referring to FIG. 1, the encoding apparatus 100 may include a motionprediction unit 111, a motion compensation unit 112, an intra-predictionunit 120, a switch 115, a subtractor 125, a transform unit 130, aquantization unit 140, an entropy encoding unit 150, a dequantizationunit 160, an inverse-transform unit 170, an adder 175, a filter unit180, and a reference picture buffer 190.

The encoding apparatus 100 may perform encoding of an input image byusing an intra mode or an inter mode or both. In addition, encodingapparatus 100 may generate a bitstream including encoded informationthrough encoding the input image, and output the generated bitstream.The generated bitstream may be stored in a computer readable recordingmedium, or may be streamed through a wired/wireless transmission medium.When an intra mode is used as a prediction mode, the switch 115 may beswitched to an intra. Alternatively, when an inter mode is used as aprediction mode, the switch 115 may be switched to an inter mode.Herein, the intra mode may mean an intra-prediction mode, and the intermode may mean an inter-prediction mode. The encoding apparatus 100 maygenerate a prediction block for an input block of the input image. Inaddition, the encoding apparatus 100 may encode a residual block using aresidual of the input block and the prediction block after theprediction block being generated. The input image may be called as acurrent image that is a current encoding target. The input block may becalled as a current block that is current encoding target, or as anencoding target block.

When a prediction mode is an intra mode, the intra-prediction unit 120may use a sample of a block that has been already encoded/decoded and isadjacent to a current block as a reference sample. The intra-predictionunit 120 may perform spatial prediction for the current block by using areference sample, or generate prediction samples of an input block byperforming spatial prediction. Herein, the intra prediction may meanintra-prediction,

When a prediction mode is an inter mode, the motion prediction unit 111may retrieve a region that best matches with an input block from areference image when performing motion prediction, and deduce a motionvector by using the retrieved region. In this case, a search region maybe used as the region. The reference image may be stored in thereference picture buffer 190. Here, when encoding/decoding for thereference image is performed, it may be stored in the reference picturebuffer 190.

The motion compensation unit 112 may generate a prediction block byperforming motion compensation for the current block using a motionvector. Herein, inter-prediction may mean inter-prediction or motioncompensation.

When the value of the motion vector is not an integer, the motionprediction unit 111 and the motion compensation unit 112 may generatethe prediction block by applying an interpolation filter to a partialregion of the reference picture. In order to perform inter-pictureprediction or motion compensation on a coding unit, it may be determinedthat which mode among a skip mode, a merge mode, an advanced motionvector prediction (AMVP) mode, and a current picture referring mode isused for motion prediction and motion compensation of a prediction unitincluded in the corresponding coding unit. Then, inter-pictureprediction or motion compensation may be differently performed dependingon the determined mode.

The subtractor 125 may generate a residual block by using a differenceof an input block and a prediction block. The residual block may becalled as a residual signal. The residual signal may mean a differencebetween an original signal and a prediction signal. In addition, theresidual signal may be a signal generated by transforming or quantizing,or transforming and quantizing a difference between the original signaland the prediction signal. The residual block may be a residual signalof a block unit.

The transform unit 130 may generate a transform coefficient byperforming transform of a residual block, and output the generatedtransform coefficient. Herein, the transform coefficient may be acoefficient value generated by performing transform of the residualblock. When a transform skip mode is applied, the transform unit 130 mayskip transform of the residual block.

A quantized level may be generated by applying quantization to thetransform coefficient or to the residual signal. Hereinafter, thequantized level may be also called as a transform coefficient inembodiments.

The quantization unit 140 may generate a quantized level by quantizingthe transform coefficient or the residual signal according to aparameter, and output the generated quantized level. Herein, thequantization unit 140 may quantize the transform coefficient by using aquantization matrix.

The entropy encoding unit 150 may generate a bitstream by performingentropy encoding according to a probability distribution on valuescalculated by the quantization unit 140 or on coding parameter valuescalculated when performing encoding, and output the generated bitstream.The entropy encoding unit 150 may perform entropy encoding of sampleinformation of an image and information for decoding an image. Forexample, the information for decoding the image may include a syntaxelement.

When entropy encoding is applied, symbols are represented so that asmaller number of bits are assigned to a symbol having a high chance ofbeing generated and a larger number of bits are assigned to a symbolhaving a low chance of being generated, and thus, the size of bit streamfor symbols to be encoded may be decreased. The entropy encoding unit150 may use an encoding method for entropy encoding such as exponentialGolomb, context-adaptive variable length coding (CAVLC),context-adaptive binary arithmetic coding (CABAC), etc. For example, theentropy encoding unit 150 may perform entropy encoding by using avariable length coding/code (VLC) table. In addition, the entropyencoding unit 150 may deduce a binarization method of a target symboland a probability model of a target symbol/bin, and perform arithmeticcoding by using the deduced binarization method, and a context model.

In order to encode a transform coefficient level(quantized level), theentropy encoding unit 150 may change a two-dimensional block formcoefficient into a one-dimensional vector form by using a transformcoefficient scanning method.

A coding parameter may include information (flag, index, etc.) such assyntax element that is encoded in an encoder and signaled to a decoder,and information derived when performing encoding or decoding. The codingparameter may mean information required when encoding or decoding animage. For example, at least one value or a combination form of aunit/block size, a unit/block depth, unit/block partition information,unit/block shape, unit/block partition structure, whether to partitionof a quad-tree form, whether to partition of a binary-tree form, apartition direction of a binary-tree form (horizontal direction orvertical direction), a partition form of a binary-tree form (symmetricpartition or asymmetric partition), whether or not a current coding unitis partitioned by ternary tree partitioning, direction (horizontal orvertical direction) of the ternary tree partitioning, type (symmetric orasymmetric type) of the ternary tree partitioning, whether a currentcoding unit is partitioned by multi-type tree partitioning, direction(horizontal or vertical direction) of the multi-type three partitioning,type (symmetric or asymmetric type) of the multi-type tree partitioning,and a tree (binary tree or ternary tree) structure of the multi-typetree partitioning, a prediction mode(intra prediction or interprediction), a luma intra-prediction mode/direction, a chromaintra-prediction mode/direction, intra partition information, interpartition information, a coding block partition flag, a prediction blockpartition flag, a transform block partition flag, a reference samplefiltering method, a reference sample filter tab, a reference samplefilter coefficient, a prediction block filtering method, a predictionblock filter tap, a prediction block filter coefficient, a predictionblock boundary filtering method, a prediction block boundary filter tab,a prediction block boundary filter coefficient, an intra-predictionmode, an inter-prediction mode, motion information, a motion vector, amotion vector difference, a reference picture index, a inter-predictionangle, an inter-prediction indicator, a prediction list utilizationflag, a reference picture list, a reference picture, a motion vectorpredictor index, a motion vector predictor candidate, a motion vectorcandidate list, whether to use a merge mode, a merge index, a mergecandidate, a merge candidate list, whether to use a skip mode, aninterpolation filter type, an interpolation filter tab, an interpolationfilter coefficient, a motion vector size, a presentation accuracy of amotion vector, a transform type, a transform size, information ofwhether or not a primary(first) transform is used, information ofwhether or not a secondary transform is used, a primary transform index,a secondary transform index, information of whether or not a residualsignal is present, a coded block pattern, a coded block flag(CBF), aquantization parameter, a quantization parameter residue, a quantizationmatrix, whether to apply an intra loop filter, an intra loop filtercoefficient, an intra loop filter tab, an intra loop filter shape/form,whether to apply a deblocking filter, a deblocking filter coefficient, adeblocking filter tab, a deblocking filter strength, a deblocking filtershape/form, whether to apply an adaptive sample offset, an adaptivesample offset value, an adaptive sample offset category, an adaptivesample offset type, whether to apply an adaptive loop filter, anadaptive loop filter coefficient, an adaptive loop filter tab, anadaptive loop filter shape/form, a binarization/inverse-binarizationmethod, a context model determining method, a context model updatingmethod, whether to perform a regular mode, whether to perform a bypassmode, a context bin, a bypass bin, a significant coefficient flag, alast significant coefficient flag, a coded flag for a unit of acoefficient group, a position of the last significant coefficient, aflag for whether a value of a coefficient is larger than 1, a flag forwhether a value of a coefficient is larger than 2, a flag for whether avalue of a coefficient is larger than 3, information on a remainingcoefficient value, a sign information, a reconstructed luma sample, areconstructed chroma sample, a residual luma sample, a residual chromasample, a luma transform coefficient, a chroma transform coefficient, aquantized luma level, a quantized chroma level, a transform coefficientlevel scanning method, a size of a motion vector search area at adecoder side, a shape of a motion vector search area at a decoder side,a number of time of a motion vector search at a decoder side,information on a CTU size, information on a minimum block size,information on a maximum block size, information on a maximum blockdepth, information on a minimum block depth, an imagedisplaying/outputting sequence, slice identification information, aslice type, slice partition information, tile identificationinformation, a tile type, tile partition information, tile groupidentification information, a tile group type, tile group partitioninformation, a picture type, a bit depth of an input sample, a bit depthof a reconstruction sample, a bit depth of a residual sample, a bitdepth of a transform coefficient, a bit depth of a quantized level, andinformation on a luma signal or information on a chroma signal may beincluded in the coding parameter.

Herein, signaling the flag or index may mean that a corresponding flagor index is entropy encoded and included in a bitstream by an encoder,and may mean that the corresponding flag or index is entropy decodedfrom a bitstream by a decoder.

When the encoding apparatus 100 performs encoding throughinter-prediction, an encoded current image may be used as a referenceimage for another image that is processed afterwards. Accordingly, theencoding apparatus 100 may reconstruct or decode the encoded currentimage, or store the reconstructed or decoded image as a reference imagein reference picture buffer 190.

A quantized level may be dequantized in the dequantization unit 160, ormay be inverse-transformed in the inverse-transform unit 170. Adequantized or inverse-transformed coefficient or both may be added witha prediction block by the adder 175. By adding the dequantized orinverse-transformed coefficient or both with the prediction block, areconstructed block may be generated. Herein, the dequantized orinverse-transformed coefficient or both may mean a coefficient on whichat least one of dequantization and inverse-transform is performed, andmay mean a reconstructed residual block.

A reconstructed block may pass through the filter unit 180. The filterunit 180 may apply at least one of a deblocking filter, a sampleadaptive offset (SAO), and an adaptive loop filter (ALF) to areconstructed sample, a reconstructed block or a reconstructed image.The filter unit 180 may be called as an in-loop filter.

The deblocking filter may remove block distortion generated inboundaries between blocks. In order to determine whether or not to applya deblocking filter, whether or not to apply a deblocking filter to acurrent block may be determined based samples included in several rowsor columns which are included in the block. When a deblocking filter isapplied to a block, another filter may be applied according to arequired deblocking filtering strength.

In order to compensate an encoding error, a proper offset value may beadded to a sample value by using a sample adaptive offset. The sampleadaptive offset may correct an offset of a deblocked image from anoriginal image by a sample unit. A method of partitioning samples of animage into a predetermined number of regions, determining a region towhich an offset is applied, and applying the offset to the determinedregion, or a method of applying an offset in consideration of edgeinformation on each sample may be used.

The adaptive loop filter may perform filtering based on a comparisonresult of the filtered reconstructed image and the original image.Samples included in an image may be partitioned into predeterminedgroups, a filter to be applied to each group may be determined, anddifferential filtering may be performed for each group. Information ofwhether or not to apply the ALF may be signaled by coding units (CUs),and a form and coefficient of the ALF to be applied to each block mayvary.

The reconstructed block or the reconstructed image having passed throughthe filter unit 180 may be stored in the reference picture buffer 190. Areconstructed block processed by the filter unit 180 may be a part of areference image. That is, a reference image is a reconstructed imagecomposed of reconstructed blocks processed by the filter unit 180. Thestored reference image may be used later in inter prediction or motioncompensation.

FIG. 2 is a block diagram showing a configuration of a decodingapparatus according to an embodiment and to which the present inventionis applied.

A decoding apparatus 200 may a decoder, a video decoding apparatus, oran image decoding apparatus.

Referring to FIG. 2, the decoding apparatus 200 may include an entropydecoding unit 210, a dequantization unit 220, an inverse-transform unit230, an intra-prediction unit 240, a motion compensation unit 250, anadder 225, a filter unit 260, and a reference picture buffer 270.

The decoding apparatus 200 may receive a bitstream output from theencoding apparatus 100. The decoding apparatus 200 may receive abitstream stored in a computer readable recording medium, or may receivea bitstream that is streamed through a wired/wireless transmissionmedium. The decoding apparatus 200 may decode the bitstream by using anintra mode or an inter mode. In addition, the decoding apparatus 200 maygenerate a reconstructed image generated through decoding or a decodedimage, and output the reconstructed image or decoded image.

When a prediction mode used when decoding is an intra mode, a switch maybe switched to an intra. Alternatively, when a prediction mode used whendecoding is an inter mode, a switch may be switched to an inter mode.

The decoding apparatus 200 may obtain a reconstructed residual block bydecoding the input bitstream, and generate a prediction block. When thereconstructed residual block and the prediction block are obtained, thedecoding apparatus 200 may generate a reconstructed block that becomes adecoding target by adding the reconstructed residual block with theprediction block. The decoding target block may be called a currentblock.

The entropy decoding unit 210 may generate symbols by entropy decodingthe bitstream according to a probability distribution. The generatedsymbols may include a symbol of a quantized level form. Herein, anentropy decoding method may be an inverse-process of the entropyencoding method described above.

In order to decode a transform coefficient level(quantized level), theentropy decoding unit 210 may change a one-directional vector formcoefficient into a two-dimensional block form by using a transformcoefficient scanning method.

A quantized level may be dequantized in the dequantization unit 220, orinverse-transformed in the inverse-transform unit 230. The quantizedlevel may be a result of dequantizing or inverse-transforming or both,and may be generated as a reconstructed residual block. Herein, thedequantization unit 220 may apply a quantization matrix to the quantizedlevel.

When an intra mode is used, the intra-prediction unit 240 may generate aprediction block by performing, for the current block, spatialprediction that uses a sample value of a block adjacent to a decodingtarget block and which has been already decoded.

When an inter mode is used, the motion compensation unit 250 maygenerate a prediction block by performing, for the current block, motioncompensation that uses a motion vector and a reference image stored inthe reference picture buffer 270.

The adder 255 may generate a reconstructed block by adding thereconstructed residual block with the prediction block. The filter unit260 may apply at least one of a deblocking filter, a sample adaptiveoffset, and an adaptive loop filter to the reconstructed block orreconstructed image. The filter unit 260 may output the reconstructedimage. The reconstructed block or reconstructed image may be stored inthe reference picture buffer 270 and used when performinginter-prediction. A reconstructed block processed by the filter unit 260may be a part of a reference image. That is, a reference image is areconstructed image composed of reconstructed blocks processed by thefilter unit 260. The stored reference image may be used later in interprediction or motion compensation.

FIG. 3 is a view schematically showing a partition structure of an imagewhen encoding and decoding the image. FIG. 3 schematically shows anexample of partitioning a single unit into a plurality of lower units.

In order to efficiently partition an image, when encoding and decoding,a coding unit (CU) may be used. The coding unit may be used as a basicunit when encoding/decoding the image. In addition, the coding unit maybe used as a unit for distinguishing an intra prediction mode and aninter prediction mode when encoding/decoding the image. The coding unitmay be a basic unit used for prediction, transform, quantization,inverse-transform, dequantization, or an encoding/decoding process of atransform coefficient.

Referring to FIG. 3, an image 300 is sequentially partitioned in alargest coding unit (LCU), and a LCU unit is determined as a partitionstructure. Herein, the LCU may be used in the same meaning as a codingtree unit (CTU). A unit partitioning may mean partitioning a blockassociated with to the unit. In block partition information, informationof a unit depth may be included. Depth information may represent anumber of times or a degree or both in which a unit is partitioned. Asingle unit may be partitioned into a plurality of lower level unitshierarchically associated with depth information based on a treestructure. In other words, a unit and a lower level unit generated bypartitioning the unit may correspond to a node and a child node of thenode, respectively. Each of partitioned lower unit may have depthinformation. Depth information may be information representing a size ofa CU, and may be stored in each CU. Unit depth represents times and/ordegrees related to partitioning a unit. Therefore, partitioninginformation of a lower-level unit may comprise information on a size ofthe lower-level unit.

A partition structure may mean a distribution of a coding unit (CU)within an LCU 310. Such a distribution may be determined according towhether or not to partition a single CU into a plurality (positiveinteger equal to or greater than 2 including 2, 4, 8, 16, etc.) of CUs.A horizontal size and a vertical size of the CU generated bypartitioning may respectively be half of a horizontal size and avertical size of the CU before partitioning, or may respectively havesizes smaller than a horizontal size and a vertical size beforepartitioning according to a number of times of partitioning. The CU maybe recursively partitioned into a plurality of CUs. By the recursivepartitioning, at least one among a height and a width of a CU afterpartitioning may decrease comparing with at least one among a height anda width of a CU before partitioning. Partitioning of the CU may berecursively performed until to a predefined depth or predefined size.For example, a depth of an LCU may be 0, and a depth of a smallestcoding unit (SCU) may be a predefined maximum depth. Herein, the LCU maybe a coding unit having a maximum coding unit size, and the SCU may be acoding unit having a minimum coding unit size as described above.Partitioning is started from the LCU 310, a CU depth increases by 1 as ahorizontal size or a vertical size or both of the CU decreases bypartitioning. For example, for each depth, a CU which is not partitionedmay have a size of 2N×2N. Also, in case of a CU which is partitioned, aCU with a size of 2N×2N may be partitioned into four CUs with a size ofN×N. A size of N may decrease to half as a depth increase by 1.

In addition, information whether or not the CU is partitioned may berepresented by using partition information of the CU. The partitioninformation may be 1-bit information. All CUs, except for a SCU, mayinclude partition information. For example, when a value of partitioninformation is a first value, the CU may not be partitioned, when avalue of partition information is a second value, the CU may bepartitioned.

Referring to FIG. 3, an LCU having a depth 0 may be a 64×64 block. 0 maybe a minimum depth. A SCU having a depth 3 may be an 8×8 block. 3 may bea maximum depth. A CU of a 32×32 block and a 16×16 block may berespectively represented as a depth 1 and a depth 2.

For example, when a single coding unit is partitioned into four codingunits, a horizontal size and a vertical size of the four partitionedcoding units may be a half size of a horizontal and vertical size of theCU before being partitioned. In one embodiment, when a coding unithaving a 32×32 size is partitioned into four coding units, each of thefour partitioned coding units may have a 16×16 size. When a singlecoding unit is partitioned into four coding units, it may be called thatthe coding unit may be partitioned into a quad-tree form.

For example, when one coding unit is partitioned into two sub-codingunits, the horizontal or vertical size (width or height) of each of thetwo sub-coding units may be half the horizontal or vertical size of theoriginal coding unit. For example, when a coding unit having a size of32×32 is vertically partitioned into two sub-coding units, each of thetwo sub-coding units may have a size of 16×32. For example, when acoding unit having a size of 8×32 is horizontally partitioned into twosub-coding units, each of the two sub-coding units may have a size of8×16. When one coding unit is partitioned into two sub-coding units, itcan be said that the coding unit is binary-partitioned or is partitionedby a binary tree partition structure.

For example, when one coding unit is partitioned into three sub-codingunits, the horizontal or vertical size of the coding unit can bepartitioned with a ratio of 1:2:1, thereby producing three sub-codingunits whose horizontal or vertical sizes are in a ratio of 1:2:1. Forexample, when a coding unit having a size of 16×32 is horizontallypartitioned into three sub-coding units, the three sub-coding units mayhave sizes of 16×8, 16×16, and 16×8 respectively, in the order from theuppermost to the lowermost sub-coding unit. For example, when a codingunit having a size of 32×32 is vertically split into three sub-codingunits, the three sub-coding units may have sizes of 8×32, 16×32, and8×32, respectively in the order from the left to the right sub-codingunit. When one coding unit is partitioned into three sub-coding units,it can be said that the coding unit is ternary-partitioned orpartitioned by a ternary tree partition structure.

In FIG. 3, a coding tree unit (CTU) 320 is an example of a CTU to whicha quad tree partition structure, a binary tree partition structure, anda ternary tree partition structure are all applied.

As described above, in order to partition the CTU, at least one of aquad tree partition structure, a binary tree partition structure, and aternary tree partition structure may be applied. Various tree partitionstructures may be sequentially applied to the CTU, according to apredetermined priority order. For example, the quad tree partitionstructure may be preferentially applied to the CTU. A coding unit thatcannot be partitioned any longer using a quad tree partition structuremay correspond to a leaf node of a quad tree. A coding unitcorresponding to a leaf node of a quad tree may serve as a root node ofa binary and/or ternary tree partition structure. That is, a coding unitcorresponding to a leaf node of a quad tree may be further partitionedby a binary tree partition structure or a ternary tree partitionstructure, or may not be further partitioned. Therefore, by preventing acoding block that results from binary tree partitioning or ternary treepartitioning of a coding unit corresponding to a leaf node of a quadtree from undergoing further quad tree partitioning, block partitioningand/or signaling of partition information can be effectively performed.

The fact that a coding unit corresponding to a node of a quad tree ispartitioned may be signaled using quad partition information. The quadpartition information having a first value (e.g., “1”) may indicate thata current coding unit is partitioned by the quad tree partitionstructure. The quad partition information having a second value (e.g.,“0”) may indicate that a current coding unit is not partitioned by thequad tree partition structure. The quad partition information may be aflag having a predetermined length (e.g., one bit).

There may not be a priority between the binary tree partitioning and theternary tree partitioning. That is, a coding unit corresponding to aleaf node of a quad tree may further undergo arbitrary partitioningamong the binary tree partitioning and the ternary tree partitioning. Inaddition, a coding unit generated through the binary tree partitioningor the ternary tree partitioning may undergo a further binary treepartitioning or a further ternary tree partitioning, or may not befurther partitioned.

A tree structure in which there is no priority among the binary treepartitioning and the ternary tree partitioning is referred to as amulti-type tree structure. A coding unit corresponding to a leaf node ofa quad tree may serve as a root node of a multi-type tree. Whether topartition a coding unit which corresponds to a node of a multi-type treemay be signaled using at least one of multi-type tree partitionindication information, partition direction information, and partitiontree information. For partitioning of a coding unit corresponding to anode of a multi-type tree, the multi-type tree partition indicationinformation, the partition direction, and the partition tree informationmay be sequentially signaled.

The multi-type tree partition indication information having a firstvalue (e.g., “1”) may indicate that a current coding unit is to undergoa multi-type tree partitioning. The multi-type tree partition indicationinformation having a second value (e.g., “0”) may indicate that acurrent coding unit is not to undergo a multi-type tree partitioning.

When a coding unit corresponding to a node of a multi-type tree isfurther partitioned by a multi-type tree partition structure, the codingunit may include partition direction information. The partitiondirection information may indicate in which direction a current codingunit is to be partitioned for the multi-type tree partitioning. Thepartition direction information having a first value (e.g., “1”) mayindicate that a current coding unit is to be vertically partitioned. Thepartition direction information having a second value (e.g., “0”) mayindicate that a current coding unit is to be horizontally partitioned.

When a coding unit corresponding to a node of a multi-type tree isfurther partitioned by a multi-type tree partition structure, thecurrent coding unit may include partition tree information. Thepartition tree information may indicate a tree partition structure whichis to be used for partitioning of a node of a multi-type tree. Thepartition tree information having a first value (e.g., “1”) may indicatethat a current coding unit is to be partitioned by a binary treepartition structure. The partition tree information having a secondvalue (e.g., “0”) may indicate that a current coding unit is to bepartitioned by a ternary tree partition structure.

The partition indication information, the partition tree information,and the partition direction information may each be a flag having apredetermined length (e.g., one bit).

At least any one of the quadtree partition indication information, themulti-type tree partition indication information, the partitiondirection information, and the partition tree information may be entropyencoded/decoded. For the entropy-encoding/decoding of those types ofinformation, information on a neighboring coding unit adjacent to thecurrent coding unit may be used. For example, there is a highprobability that the partition type (the partitioned or non-partitioned,the partition tree, and/or the partition direction) of a leftneighboring coding unit and/or an upper neighboring coding unit of acurrent coding unit is similar to that of the current coding unit.Therefore, context information for entropy encoding/decoding of theinformation on the current coding unit may be derived from theinformation on the neighboring coding units. The information on theneighboring coding units may include at least any one of quad partitioninformation, multi-type tree partition indication information, partitiondirection information, and partition tree information.

As another example, among binary tree partitioning and ternary treepartitioning, binary tree partitioning may be preferentially performed.That is, a current coding unit may primarily undergo binary treepartitioning, and then a coding unit corresponding to a leaf node of abinary tree may be set as a root node for ternary tree partitioning. Inthis case, neither quad tree partitioning nor binary tree partitioningmay not be performed on the coding unit corresponding to a node of aternary tree.

A coding unit that cannot be partitioned by a quad tree partitionstructure, a binary tree partition structure, and/or a ternary treepartition structure becomes a basic unit for coding, prediction and/ortransformation. That is, the coding unit cannot be further partitionedfor prediction and/or transformation. Therefore, the partition structureinformation and the partition information used for partitioning a codingunit into prediction units and/or transformation units may not bepresent in a bit stream.

However, when the size of a coding unit (i.e., a basic unit forpartitioning) is larger than the size of a maximum transformation block,the coding unit may be recursively partitioned until the size of thecoding unit is reduced to be equal to or smaller than the size of themaximum transformation block. For example, when the size of a codingunit is 64×64 and when the size of a maximum transformation block is32×32, the coding unit may be partitioned into four 32×32 blocks fortransformation. For example, when the size of a coding unit is 32×64 andthe size of a maximum transformation block is 32×32, the coding unit maybe partitioned into two 32×32 blocks for the transformation. In thiscase, the partitioning of the coding unit for transformation is notsignaled separately, and may be determined through comparison betweenthe horizontal or vertical size of the coding unit and the horizontal orvertical size of the maximum transformation block. For example, when thehorizontal size (width) of the coding unit is larger than the horizontalsize (width) of the maximum transformation block, the coding unit may bevertically bisected. For example, when the vertical size (length) of thecoding unit is larger than the vertical size (length) of the maximumtransformation block, the coding unit may be horizontally bisected.

Information of the maximum and/or minimum size of the coding unit andinformation of the maximum and/or minimum size of the transformationblock may be signaled or determined at an upper level of the codingunit. The upper level may be, for example, a sequence level, a picturelevel, a slice level, a tile group level, a tile level, or the like. Forexample, the minimum size of the coding unit may be determined to be4×4. For example, the maximum size of the transformation block may bedetermined to be 64×64. For example, the minimum size of thetransformation block may be determined to be 4×4.

Information of the minimum size (quad tree minimum size) of a codingunit corresponding to a leaf node of a quad tree and/or information ofthe maximum depth (the maximum tree depth of a multi-type tree) from aroot node to a leaf node of the multi-type tree may be signaled ordetermined at an upper level of the coding unit. For example, the upperlevel may be a sequence level, a picture level, a slice level, a tilegroup level, a tile level, or the like. Information of the minimum sizeof a quad tree and/or information of the maximum depth of a multi-typetree may be signaled or determined for each of an intra-picture sliceand an inter-picture slice.

Difference information between the size of a CTU and the maximum size ofa transformation block may be signaled or determined at an upper levelof the coding unit. For example, the upper level may be a sequencelevel, a picture level, a slice level, a tile group level, a tile level,or the like. Information of the maximum size of the coding unitscorresponding to the respective nodes of a binary tree (hereinafter,referred to as a maximum size of a binary tree) may be determined basedon the size of the coding tree unit and the difference information. Themaximum size of the coding units corresponding to the respective nodesof a ternary tree (hereinafter, referred to as a maximum size of aternary tree) may vary depending on the type of slice. For example, foran intra-picture slice, the maximum size of a ternary tree may be 32×32.For example, for an inter-picture slice, the maximum size of a ternarytree may be 128×128. For example, the minimum size of the coding unitscorresponding to the respective nodes of a binary tree (hereinafter,referred to as a minimum size of a binary tree) and/or the minimum sizeof the coding units corresponding to the respective nodes of a ternarytree (hereinafter, referred to as a minimum size of a ternary tree) maybe set as the minimum size of a coding block.

As another example, the maximum size of a binary tree and/or the maximumsize of a ternary tree may be signaled or determined at the slice level.Alternatively, the minimum size of the binary tree and/or the minimumsize of the ternary tree may be signaled or determined at the slicelevel.

Depending on size and depth information of the above-described variousblocks, quad partition information, multi-type tree partition indicationinformation, partition tree information and/or partition directioninformation may be included or may not be included in a bit stream.

For example, when the size of the coding unit is not larger than theminimum size of a quad tree, the coding unit does not contain quadpartition information. Thus, the quad partition information may bededuced from a second value.

For example, when the sizes (horizontal and vertical sizes) of a codingunit corresponding to a node of a multi-type tree are larger than themaximum sizes (horizontal and vertical sizes) of a binary tree and/orthe maximum sizes (horizontal and vertical sizes) of a ternary tree, thecoding unit may not be binary-partitioned or ternary-partitioned.Accordingly, the multi-type tree partition indication information maynot be signaled but may be deduced from a second value.

Alternatively, when the sizes (horizontal and 23 vertical sizes) of acoding unit corresponding to a node of a multi-type tree are the same asthe maximum sizes (horizontal and vertical sizes) of a binary treeand/or are two times as large as the maximum sizes (horizontal andvertical sizes) of a ternary tree, the coding unit may not be furtherbinary-partitioned or ternary-partitioned. Accordingly, the multi-typetree partition indication information may not be signaled but be derivedfrom a second value. This is because when a coding unit is partitionedby a binary tree partition structure and/or a ternary tree partitionstructure, a coding unit smaller than the minimum size of a binary treeand/or the minimum size of a ternary tree is generated.

Alternatively, the binary tree partitioning or the ternary treepartitioning may be limited on the basis of the size of a virtualpipeline data unit (hereinafter, a pipeline buffer size). For example,when the coding unit is divided into sub-coding units which do not fitthe pipeline buffer size by the binary tree partitioning or the ternarytree partitioning, the corresponding binary tree partitioning or ternarytree partitioning may be limited. The pipeline buffer size may be thesize of the maximum transform block (e.g., 64×64). For example, when thepipeline buffer size is 64×64, the division below may be limited.

-   -   N×M (N and/or M is 128) Ternary tree partitioning for coding        units    -   128×N (N<=64) Binary tree partitioning in horizontal direction        for coding units    -   N×128 (N<=64) Binary tree partitioning in vertical direction for        coding units

Alternatively, when the depth of a coding unit corresponding to a nodeof a multi-type tree is equal to the maximum depth of the multi-typetree, the coding unit may not be further binary-partitioned and/orternary-partitioned. Accordingly, the multi-type tree partitionindication information may not be signaled but may be deduced from asecond value.

Alternatively, only when at least one of vertical direction binary treepartitioning, horizontal direction binary tree partitioning, verticaldirection ternary tree partitioning, and horizontal direction ternarytree partitioning is possible for a coding unit corresponding to a nodeof a multi-type tree, the multi-type tree partition indicationinformation may be signaled. Otherwise, the coding unit may not bebinary-partitioned and/or ternary-partitioned. Accordingly, themulti-type tree partition indication information may not be signaled butmay be deduced from a second value.

Alternatively, only when both of the vertical direction binary treepartitioning and the horizontal direction binary tree partitioning orboth of the vertical direction ternary tree partitioning and thehorizontal direction ternary tree partitioning are possible for a codingunit corresponding to a node of a multi-type tree, the partitiondirection information may be signaled. Otherwise, the partitiondirection information may not be signaled but may be derived from avalue indicating possible partitioning directions.

Alternatively, only when both of the vertical direction binary treepartitioning and the vertical direction ternary tree partitioning orboth of the horizontal direction binary tree partitioning and thehorizontal direction ternary tree partitioning are possible for a codingtree corresponding to a node of a multi-type tree, the partition treeinformation may be signaled. Otherwise, the partition tree informationmay not be signaled but be deduced from a value indicating a possiblepartitioning tree structure.

FIG. 4 is a view showing an intra-prediction process.

Arrows from center to outside in FIG. 4 may represent predictiondirections of intra prediction modes.

Intra encoding and/or decoding may be performed by using a referencesample of a neighbor block of the current block. A neighbor block may bea reconstructed neighbor block. For example, intra encoding and/ordecoding may be performed by using an encoding parameter or a value of areference sample included in a reconstructed neighbor block.

A prediction block may mean a block generated by performing intraprediction. A prediction block may correspond to at least one among CU,PU and TU. A unit of a prediction block may have a size of one among CU,PU and TU. A prediction block may be a square block having a size of2×2, 4×4, 16×16, 32×32 or 64×64 etc. or may be a rectangular blockhaving a size of 2×8, 4×8, 2×16, 4×16 and 8×16 etc.

Intra prediction may be performed according to intra prediction mode forthe current block. The number of intra prediction modes which thecurrent block may have may be a fixed value and may be a valuedetermined differently according to an attribute of a prediction block.For example, an attribute of a prediction block may comprise a size of aprediction block and a shape of a prediction block, etc.

The number of intra-prediction modes may be fixed to N regardless of ablock size. Or, the number of intra prediction modes may be 3, 5, 9, 17,34, 35, 36, 65, or 67 etc. Alternatively, the number of intra-predictionmodes may vary according to a block size or a color component type orboth. For example, the number of intra prediction modes may varyaccording to whether the color component is a luma signal or a chromasignal. For example, as a block size becomes large, a number ofintra-prediction modes may increase. Alternatively, a number ofintra-prediction modes of a luma component block may be larger than anumber of intra-prediction modes of a chroma component block.

An intra-prediction mode may be a non-angular mode or an angular mode.The non-angular mode may be a DC mode or a planar mode, and the angularmode may be a prediction mode having a specific direction or angle. Theintra-prediction mode may be expressed by at least one of a mode number,a mode value, a mode numeral, a mode angle, and mode direction. A numberof intra-prediction modes may be M, which is larger than 1, includingthe non-angular and the angular mode. In order to intra-predict acurrent block, a step of determining whether or not samples included ina reconstructed neighbor block may be used as reference samples of thecurrent block may be performed. When a sample that is not usable as areference sample of the current block is present, a value obtained byduplicating or performing interpolation on at least one sample valueamong samples included in the reconstructed neighbor block or both maybe used to replace with a non-usable sample value of a sample, thus thereplaced sample value is used as a reference sample of the currentblock.

FIG. 7 is a diagram illustrating reference samples capable of being usedfor intra prediction.

As shown in FIG. 7, at least one of the reference sample line 0 to thereference sample line 3 may be used for intra prediction of the currentblock. In FIG. 7, the samples of a segment A and a segment F may bepadded with the samples closest to a segment B and a segment E,respectively, instead of retrieving from the reconstructed neighboringblock. Index information indicating the reference sample line to be usedfor intra prediction of the current block may be signaled. When theupper boundary of the current block is the boundary of the CTU, only thereference sample line 0 may be available. Therefore, in this case, theindex information may not be signaled. When a reference sample lineother than the reference sample line 0 is used, filtering for aprediction block, which will be described later, may not be performed.

When intra-predicting, a filter may be applied to at least one of areference sample and a prediction sample based on an intra-predictionmode and a current block size.

In case of a planar mode, when generating a prediction block of acurrent block, according to a position of a prediction target samplewithin a prediction block, a sample value of the prediction targetsample may be generated by using a weighted sum of an upper and leftside reference sample of a current sample, and a right upper side andleft lower side reference sample of the current block. In addition, incase of a DC mode, when generating a prediction block of a currentblock, an average value of upper side and left side reference samples ofthe current block may be used. In addition, in case of an angular mode,a prediction block may be generated by using an upper side, a left side,a right upper side, and/or a left lower side reference sample of thecurrent block. In order to generate a prediction sample value,interpolation of a real number unit may be performed.

In the case of intra prediction between color components, a predictionblock for the current block of the second color component may begenerated on the basis of the corresponding reconstructed block of thefirst color component. For example, the first color component may be aluma component, and the second color component may be a chromacomponent. For intra prediction between color components, the parametersof the linear model between the first color component and the secondcolor component may be derived on the basis of the template. Thetemplate may 23 include upper and/or left neighboring samples of thecurrent block and upper and/or left neighboring samples of thereconstructed block of the first color component corresponding thereto.For example, the parameters of the linear model may be derived using asample value of a first color component having a maximum value amongsamples in a template and a sample value of a second color componentcorresponding thereto, and a sample value of a first color componenthaving a minimum value among samples in the template and a sample valueof a second color component corresponding thereto. When the parametersof the linear model are derived, a corresponding reconstructed block maybe applied to the linear model to generate a prediction block for thecurrent block. According to a video format, subsampling may be performedon the neighboring samples of the reconstructed block of the first colorcomponent and the corresponding reconstructed block. For example, whenone sample of the second color component corresponds to four samples ofthe first color component, four samples of the first color component maybe sub-sampled to compute one corresponding sample. In this case, theparameter derivation of the linear model and intra prediction betweencolor components may be performed on the basis of the correspondingsub-sampled samples. Whether or not to perform intra prediction betweencolor components and/or the range of the template may be signaled as theintra prediction mode.

The current block may be partitioned into two or four sub-blocks in thehorizontal or vertical direction. The partitioned sub-blocks may besequentially reconstructed. That is, the intra prediction may beperformed on the sub-block to generate the sub-prediction block. Inaddition, dequantization and/or inverse transform may be performed onthe sub-blocks to generate sub-residual blocks. A reconstructedsub-block may be generated by adding the sub-prediction block to thesub-residual block. The reconstructed sub-block may be used as areference sample for intra prediction of the sub-blocks. The sub-blockmay be a block including a predetermined number (for example, 16) ormore samples. Accordingly, for example, when the current block is an 8×4block or a 4×8 block, the current block may be partitioned into twosub-blocks. Also, when the current block is a 4×4 block, the currentblock may not be partitioned into sub-blocks. When the current block hasother sizes, the current block may be partitioned into four sub-blocks.Information on whether or not to perform the intra prediction based onthe sub-blocks and/or the partitioning direction (horizontal orvertical) may be signaled. The intra prediction based on the sub-blocksmay be limited to be performed only when reference sample line 0 isused. When the intra prediction based on the sub-block is performed,filtering for the prediction block, which will be described later, maynot be performed.

The final prediction block may be generated by performing filtering onthe prediction block that is intra-predicted. The filtering may beperformed by applying predetermined weights to the filtering targetsample, the left reference sample, the upper reference sample, and/orthe upper left reference sample. The weight and/or the reference sample(range, position, etc.) used for the filtering may be determined on thebasis of at least one of a block size, an intra prediction mode, and aposition of the filtering target sample in the prediction block. Thefiltering may be performed only in the case of a predetermined intraprediction mode (e.g., DC, planar, vertical, horizontal, diagonal,and/or adjacent diagonal modes). The adjacent diagonal mode may be amode in which k is added to or subtracted from the diagonal mode. Forexample, k may be a positive integer of 8 or less.

An intra-prediction mode of a current block may be entropyencoded/decoded by predicting an intra-prediction mode of a blockpresent adjacent to the current block. When intra-prediction modes ofthe current block and the neighbor block are identical, information thatthe intra-prediction modes of the current block and the neighbor blockare identical may be signaled by using predetermined flag information.In addition, indicator information of an intra-prediction mode that isidentical to the intra-prediction mode of the current block amongintra-prediction modes of a plurality of neighbor blocks may besignaled. When intra-prediction modes of the current block and theneighbor block are different, intra-prediction mode information of thecurrent block may be entropy encoded/decoded by performing entropyencoding/decoding based on the intra-prediction mode of the neighborblock.

FIG. 5 is a diagram illustrating an embodiment of an inter-pictureprediction process.

In FIG. 5, a rectangle may represent a picture. In FIG. 5, an arrowrepresents a prediction direction. Pictures may be categorized intointra pictures (I pictures), predictive pictures (P pictures), andBi-predictive pictures (B pictures) according to the encoding typethereof.

The I picture may be encoded through intra-prediction without requiringinter-picture prediction. The P picture may be encoded throughinter-picture prediction by using a reference picture that is present inone direction (i.e., forward direction or backward direction) withrespect to a current block. The B picture may be encoded throughinter-picture prediction by using reference pictures that are preset intwo directions (i.e., forward direction and backward direction) withrespect to a current block. When the inter-picture prediction is used,the encoder may perform inter-picture prediction or motion compensationand the decoder may perform the corresponding motion compensation.

Hereinbelow, an embodiment of the inter-picture prediction will bedescribed in detail.

The inter-picture prediction or motion compensation may be performedusing a reference picture and motion information.

Motion information of a current block may be derived duringinter-picture prediction by each of the encoding apparatus 100 and thedecoding apparatus 200. The motion information of the current block maybe derived by using motion information of a reconstructed neighboringblock, motion information of a collocated block (also referred to as acol block or a co-located block), and/or a block adjacent to theco-located block. The co-located block may mean a block that is locatedspatially at the same position as the current block, within a previouslyreconstructed collocated picture (also referred to as a col picture or aco-located picture). The co-located picture may be one picture among oneor more reference pictures included in a reference picture list.

The derivation method of the motion information may be differentdepending on the prediction mode of the current block. For example, aprediction mode applied for inter prediction includes an AMVP mode, amerge mode, a skip mode, a merge mode with a motion vector difference, asubblock merge mode, a triangle partition mode, an inter-intracombination prediction mode, affine mode, and the like. Herein, themerge mode may be referred to as a motion merge mode.

For example, when the AMVP is used as the prediction mode, at least oneof motion vectors of the reconstructed neighboring blocks, motionvectors of the co-located blocks, motion vectors of blocks adjacent tothe co-located blocks, and a (0, 0) motion vector may be determined asmotion vector candidates for the current block, and a motion vectorcandidate list is generated by using the emotion vector candidates. Themotion vector candidate of the current block can be derived by using thegenerated motion vector candidate list. The motion information of thecurrent block may be determined based on the derived motion vectorcandidate. The motion vectors of the collocated blocks or the motionvectors of the blocks adjacent to the collocated blocks may be referredto as temporal motion vector candidates, and the motion vectors of thereconstructed neighboring blocks may be referred to as spatial motionvector candidates.

The encoding apparatus 100 may calculate a motion vector difference(MVD) between the motion vector of the current block and the motionvector candidate and may perform entropy encoding on the motion vectordifference (MVD). In addition, the encoding apparatus 100 may performentropy encoding on a motion vector candidate index and generate abitstream. The motion vector candidate index may indicate an optimummotion vector candidate among the motion vector candidates included inthe motion vector candidate list. The decoding apparatus may performentropy decoding on the motion vector candidate index included in thebitstream and may select a motion vector candidate of a decoding targetblock from among the motion vector candidates included in the motionvector candidate list by using the entropy-decoded motion vectorcandidate index. In addition, the decoding apparatus 200 may add theentropy-decoded MVD and the motion vector candidate extracted throughthe entropy decoding, thereby deriving the motion vector of the decodingtarget block.

Meanwhile, the coding apparatus 100 may perform entropy-coding onresolution information of the calculated MVD. The decoding apparatus 200may adjust the resolution of the entropy-decoded MVD using the MVDresolution information.

Meanwhile, the coding apparatus 100 calculates a motion vectordifference (MVD) between a motion vector and a motion vector candidatein the current block on the basis of an affine model, and performsentropy-coding on the MVD. The decoding apparatus 200 derives a motionvector on a per sub-block basis by deriving an affine control motionvector of a decoding target block through the sum of the entropy-decodedMVD and an affine control motion vector candidate.

The bitstream may include a reference picture index indicating areference picture. The reference picture index may be entropy-encoded bythe encoding apparatus 100 and then signaled as a bitstream to thedecoding apparatus 200. The decoding apparatus 200 may generate aprediction block of the decoding target block based on the derivedmotion vector and the reference picture index information.

Another example of the method of deriving the motion information of thecurrent may be the merge mode. The merge mode may mean a method ofmerging motion of a plurality of blocks. The merge mode may mean a modeof deriving the motion information of the current block from the motioninformation of the neighboring blocks. When the merge mode is applied,the merge candidate list may be generated using the motion informationof the reconstructed neighboring blocks and/or the motion information ofthe collocated blocks. The motion information may include at least oneof a motion vector, a reference picture index, and an inter-pictureprediction indicator. The prediction indicator may indicateone-direction prediction (L0 prediction or L1 prediction) ortwo-direction predictions (L0 prediction and L1 prediction).

The merge candidate list may be a list of motion information stored. Themotion information included in the merge candidate list may be at leastone of motion information (spatial merge candidate) of a neighboringblock adjacent to the current block, motion information (temporal mergecandidate) of the collocated block of the current block in the referencepicture, new motion information generated by a combination of the motioninformation exiting in the merge candidate list, motion information(history-based merge candidate) of the block that is encoded/decodedbefore the current block, and zero merge candidate.

The encoding apparatus 100 may generate a bitstream by performingentropy encoding on at least one of a merge flag and a merge index andmay signal the bitstream to the decoding apparatus 200. The merge flagmay be information indicating whether or not to perform the merge modefor each block, and the merge index may be information indicating thatwhich neighboring block, among the neighboring blocks of the currentblock, is a merge target block. For example, the neighboring blocks ofthe current block may include a left neighboring block on the left sideof the current block, an upper neighboring block disposed above thecurrent block, and a temporal neighboring block temporally adjacent tothe current block.

Meanwhile, the coding apparatus 100 performs entropy-coding on thecorrection information for correcting the motion vector among the motioninformation of the merge candidate and signals the same to the decodingapparatus 200. The decoding apparatus 200 can correct the motion vectorof the merge candidate selected by the merge index on the basis of thecorrection information. Here, the correction information may include atleast one of information on whether or not to perform the correction,correction direction information, and correction size information. Asdescribed above, the prediction mode that corrects the motion vector ofthe merge candidate on the basis of the signaled correction informationmay be referred to as a merge mode having the motion vector difference.

The skip mode may be a mode in which the motion information of theneighboring block is applied to the current block as it is. When theskip mode is applied, the encoding apparatus 100 may perform entropyencoding on information of the fact that the motion information of whichblock is to be used as the motion information of the current block togenerate a bit stream, and may signal the bitstream to the decodingapparatus 200. The encoding apparatus 100 may not signal a syntaxelement regarding at least any one of the motion vector differenceinformation, the encoding block flag, and the transform coefficientlevel to the decoding apparatus 200.

The subblock merge mode may mean a mode that derives the motioninformation in units of sub-blocks of a coding block (CU). When thesubblock merge mode is applied, a subblock merge candidate list may begenerated using motion information (sub-block based temporal mergecandidate) of the sub-block collocated to the current sub-block in thereference image and/or an affine control point motion vector mergecandidate.

The triangle partition mode may mean a mode that derives motioninformation by partitioning the current block into diagonal directions,derives each prediction sample using each of the derived motioninformation, and derives the prediction sample of the current block byweighting each of the derived prediction samples.

The inter-intra combined prediction mode may mean a mode that derives aprediction sample of the current block by weighting a prediction samplegenerated by inter prediction and a prediction sample generated by intraprediction.

The decoding apparatus 200 may correct the derived motion information byitself. The decoding apparatus 200 may search the predetermined regionon the basis of the reference block indicated by the derived motioninformation and derive the motion information having the minimum SAD asthe corrected motion information.

The decoding apparatus 200 may compensate a prediction sample derivedvia inter prediction using an optical flow.

FIG. 6 is a diagram illustrating a transform and quantization process.

As illustrated in FIG. 6, a transform and/or quantization process isperformed on a residual signal to generate a quantized level signal. Theresidual signal is a difference between an original block and aprediction block (i.e., an intra prediction block or an inter predictionblock). The prediction block is a block generated through intraprediction or inter prediction. The transform may be a primarytransform, a secondary transform, or both. The primary transform of theresidual signal results in transform coefficients, and the secondarytransform of the transform coefficients results in secondary transformcoefficients.

At least one scheme selected from among various transform schemes whichare preliminarily defined is used to perform the primary transform. Forexample, examples of the predefined transform schemes include discretecosine transform (DCT), discrete sine transform (DST), andKarhunen-Loéve transform (KLT). The transform coefficients generatedthrough the primary transform may undergo the secondary transform. Thetransform schemes used for the primary transform and/or the secondarytransform may be determined according to coding parameters of thecurrent block and/or neighboring blocks of the current block.Alternatively, transform information indicating the transform scheme maybe signaled. The DCT-based transform may include, for example, DCT-2,DCT-8, and the like. The DST-based transform may include, for example,DST-7.

A quantized-level signal (quantization coefficients) may be generated byperforming quantization on the residual signal or a result of performingthe primary transform and/or the secondary transform. The quantizedlevel signal may be scanned according to at least one of a diagonalup-right scan, a vertical scan, and a horizontal scan, depending on anintra prediction mode of a block or a block size/shape. For example, asthe coefficients are scanned in a diagonal up-right scan, thecoefficients in a block form change into a one-dimensional vector form.Aside from the diagonal up-right scan, the horizontal scan ofhorizontally scanning a two-dimensional block form of coefficients orthe vertical scan of vertically scanning a two-dimensional block form ofcoefficients may be used depending on the intra prediction mode and/orthe size of a transform block. The scanned quantized-level coefficientsmay be entropy-encoded to be inserted into a bitstream.

A decoder entropy-decodes the bitstream to obtain the quantized-levelcoefficients. The quantized-level coefficients may be arranged in atwo-dimensional block form through inverse scanning. For the inversescanning, at least one of a diagonal up-right scan, a vertical scan, anda horizontal scan may be used.

The quantized-level coefficients may then be dequantized, then besecondary-inverse-transformed as necessary, and finally beprimary-inverse-transformed as necessary to generate a reconstructedresidual signal.

Inverse mapping in a dynamic range may be performed for a luma componentreconstructed through intra prediction or inter prediction beforein-loop filtering. The dynamic range may be divided into 16 equal piecesand the mapping function for each piece may be signaled. The mappingfunction may be signaled at a slice level or a tile group level. Aninverse mapping function for performing the inverse mapping may bederived on the basis of the mapping function. In-loop filtering,reference picture storage, and motion compensation are performed in aninverse mapped region, and a prediction block generated through interprediction is converted into a mapped region via mapping using themapping function, and then used for generating the reconstructed block.However, since the intra prediction is performed in the mapped region,the prediction block generated via the intra prediction may be used forgenerating the reconstructed block without mapping/inverse mapping.

When the current block is a residual block of a chroma component, theresidual block may be converted into an inverse mapped region byperforming scaling on the chroma component of the mapped region. Theavailability of the scaling may be signaled at the slice level or thetile group level. The scaling may be applied only when the mapping forthe luma component is available and the division of the luma componentand the division of the chroma component follow the same tree structure.The scaling may be performed on the basis of an average of sample valuesof a luma prediction block corresponding to the color difference block.In this case, when the current block uses inter prediction, the lumaprediction block may mean a mapped luma prediction block. A valuenecessary for the scaling may be derived by referring to a lookup tableusing an index of a piece to which an average of sample values of a lumaprediction block belongs. Finally, by scaling the residual block usingthe derived value, the residual block may be switched to the inversemapped region. Then, chroma component block restoration, intraprediction, inter prediction, in-loop filtering, and reference picturestorage may be performed in the inverse mapped area.

Information indicating whether the mapping/inverse mapping of the lumacomponent and chroma component is available may be signaled through aset of sequence parameters.

The prediction block of the current block may be generated on the basisof a block vector indicating a displacement between the current blockand the reference block in the current picture. In this way, aprediction mode for generating a prediction block with reference to thecurrent picture is referred to as an intra block copy (IBC) mode. TheIBC mode may be applied to M×N (M<=64, N<=64) coding units. The IBC modemay include a skip mode, a merge mode, an AMVP mode, and the like. Inthe case of a skip mode or a merge mode, a merge candidate list isconstructed, and the merge index is signaled so that one merge candidatemay be specified. The block vector of the specified merge candidate maybe used as a block vector of the current block. The merge candidate listmay include at least one of a spatial candidate, a history-basedcandidate, a candidate based on an average of two candidates, and azero-merge candidate. In the case of an AMVP mode, the difference blockvector may be signaled. In addition, the prediction block vector may bederived from the left neighboring block and the upper neighboring blockof the current block. The index on which neighboring block to use may besignaled. The prediction block in the IBC mode is included in thecurrent CTU or the left CTU and limited to a block in the alreadyreconstructed area. For example, a value of the block vector may belimited such that the prediction block of the current block ispositioned in an area of three 64×64 blocks preceding the 64×64 block towhich the current block belongs in the coding/decoding order. Bylimiting the value of the block vector in this way, memory consumptionand device complexity according to the IBC mode implementation may bereduced.

Hereinafter, an image decoding/encoding method will be describedaccording to an embodiment of the present invention.

There are a variety of encoding techniques that may be used to encode animage. In addition, a particular technique may be advantageous overother techniques depending on the nature of the image to be encoded.

Thus, the encoder may adaptively determine whether or not to use aplurality of various encoding techniques for the corresponding block,thereby performing the most advantageous encoding. Among variousselectable techniques, in order to select the most advantageous codingtechnique for the corresponding block, the encoder may generally performrate-distortion optimization (RDO).

Since the encoder cannot know in advance which of the various encodingdecisions that may be selected for encoding an image is the best at aparticular side (e.g., rate-distortion side), the encoder performsencoding (or simplified encoding) on each of the possible combinationsof encoding decisions to calculate rate-distortion values for thecombinations, and then determines an encoding decision having thesmallest rate-distortion value among the rate-distortion calculations asa final encoding decision for the block.

In addition, the encoder records the encoding decision in a bitstream,so that the decoder may read (parse) the same and perform an exactreverse process corresponding to the encoding, to make it possible toperform decoding. Here, information about the encoding decision may bereferred to as encoding decision information.

Since each channel such as YCbCr, RGB, XYZ, etc. of the image does notalways have the same property between the channels, making independentencoding decisions for each channel is more effective in terms ofimproving compression ratio.

In addition, since channels often have different block structures fromeach other (where blocks include both square or rectangular shapes), aneffective transform method may be determined through a process such asRDO among transforms (called “multiple transform”) having various sizesor various kernel functions. In addition, whether the transform isperformed on a block to be encoded simultaneously with multipletransform selection may be determined. That is, it is determined whetherthe transform is performed for each transform block, and transform skipinformation may be recorded in the bitstream for each color channel (oras specific channel information).

In the case where a variation of the spatial pixel value in thecorresponding image block to be compressed is very large, or inparticular, the variation is locally limited, the degree of image energyconcentration at low frequency is not large and the transformcoefficient of the high frequency region having a relatively large valuemay occur even when the transform is applied. Therefore, when applyingthe transform and quantization technique that mainly maintains the lowfrequency signal components through the quantization process after thetransform and removes the high frequency signal components or reducesthe amount of data by applying quantization strongly, serious imagequality degradation may occur. In particular, this problem is large whenthe variation in the image value is concentrated at a place where thevariation in the spatial pixel value is large or locally limited.

In order to solve this problem, instead of performing uniform transformon the image block, a method of performing direct encoding of pixelvalues in the spatial domain without transform may be used.

According to this technique, it is determined whether the transform isskipped for each transform block, and encoding may be performed byperforming or skipping transform according to the determination. Theproblem may be solved by inserting information indicating transform skipinto the bit stream to instruct the decoder whether to perform transformon the corresponding block. Herein, the information indicating whetherto perform the transform may be referred to as transform skipinformation.

The transform skip information may be transmitted for each of aluminance (luma or Y channel) signal and a chrominance (chroma or Cbchannel/Cr channel) signal, that is, channels Y, Cb, and Cr. The decodermay perform or skip transform for the corresponding block according tothe value of transform skip information for each channel parsed from thebitstream.

However, the existing technology has always had a problem of determiningwhether perform or skip transform for each transform block. That is,since it is determined whether to skip/perform transform simultaneouslyfor both the horizontal and vertical directions of the transform block,there is a problem that it is difficult to select an efficient transformmethod. That is, it is impossible to perform or skip transformindependently for each of the horizontal and longitudinal directions ofthe transform block. In addition, since the luma channel undergoesmultiple transform selection, the overhead required to find the optimaltransform method is larger. In addition, such a reduced compressionratio inevitably causes a problem that degrades the quality of thecompressed image.

Meanwhile, the present invention may apply a horizontal transform and avertical transform to both luma and chroma channels in a transformprocess when performing image encoding, by applying a variety oftechniques, such as transform encoding using predictive encoding andtransform to a high resolution image such as 4K or 8K.

In addition, the order of reading (parsing) the transform skipinformation and the multiple transform information may be effectivelydetermined, thereby providing more efficient compression.

According to the present invention, transform skip and multipletransform selection may be performed even when the luma channel and thechroma channel have the same block partition structure to each other orwhen the luma channel and the chroma channel has the block partitionstructures different from each other.

By using the technique of the present invention, it is possible toresolve a problem that the reduction of compression ratio and the imagequality occurs when the existing technique is applied.

In general, when transform technique is applied to an image in which thedegree of change in pixel values is spatially concentrated, energy isnot concentrated toward low frequencies, but a large number of highfrequency components are generated due to a higher change in pixelvalues, whereby it is sometimes disadvantageous to perform transformencoding in terms of compression ratio. In this case, when the transformis performed, a problem of serious deterioration may occur.

Therefore, the present invention selectively applies transform ortransform skip to each of the horizontal and vertical directions,whereby there is an effect of increasing the compression ratio whileobtaining good image quality.

Meanwhile, as the resolution of the image increases, it is sometimesadvantageous in terms of compression ratio to use a larger transformblock size during the transform. In addition, the combination of a typeof transform and the horizontal and vertical sizes of the transformblock may be used in various ways. In addition, each color channel ofthe image may not have the same block structure, or may not have thesame transform.

The transform used for image compression may require a 2D transform whenthe transform block is a 2D signal. Meanwhile, since applying the 2Dtransform is not advantageous in terms of calculation amount, aseparable transform may be used in general. The separable transform issuch that each row data of a two-dimensional transform block isone-dimensionally transformed in one direction (for example, horizontaldirection), and each column data is one-dimensionally transformed inanother direction (for example, vertical direction) for data thusobtained, thereby performing two-dimensional transform. This is possiblebecause the kernel function of transform is separable. Since such aseparable transform is used, it is possible to reduce computationcomplexity by performing a simpler one-dimensional transform twice,without a need for a large amount of computation when performing atwo-dimensional transform.

As the size of the transform block is larger, the transform gain isincreased, and when the size of the transform block is small, a case mayoccur in which there is not a big difference in terms of compressionefficiency regardless of whether the transform is performed or not.

In this respect, it is advantageous to set the size of the transformblock as large as possible to obtain higher transform gain. However,when there are many pixels with a variance of discontinuous values inone transform block (for example, when sharp edges are included), it ismore advantageous in terms of compression efficiency that block isdivided into two or more transform blocks to allow each block to beindependently transformed so that the discontinuity is not included,rather than setting the transform block to be large as one block. Forthis reason, in the encoder, it is important to set the size of thetransform block appropriately so that the size of the transform block isset to be as large as possible and the discontinuity causing a reductionin compression efficiency is not included, in order to obtain the bestcompression ratio.

In consideration of the above, it is possible to use a technique ofperforming the transform by determining a transform block size so thatthe corresponding block is an N×N block having an appropriate value whenan N×N DCT (N=4, 8, and 16) is provided. In addition, as shown in 8×8DCT in MPEG-1 and MPEG-2, 4×4 DCT in H.264/AVC, N×N DCT in HEVC (N=4, 8,16), and the like, the transform block used in image compression up tonow has been a square. That is, the horizontal and vertical sizes of thetransform block were the same.

However, the transform block does not necessarily have to be a square.That is, the horizontal and vertical sizes may be different from eachother. Therefore, when a transform block having an appropriate size isdetermined for a given image, the image may be divided into squaretransform blocks having the same horizontal and vertical size. However,in some cases, the image may be divided into transform blocks havingrectangles having horizontal and vertical sizes different from eachother.

Meanwhile, when a large number of discontinuities are included in theimage, it may be advantageous in this aspect to set the size of thetransform block as small as possible so that many discontinuities arenot included in the same block. However, when the transform block is setsmall, sometimes a case occurs in which the transform gain may be smalland the signaling overhead for indicating the transform block may belarger than the transform gain. Therefore, in this case, it may beadvantageous to perform encoding in pixel space without performing thetransform. In consideration of this aspect, the above-describedtransform skip technique may be used. The transform skip may mean thatwhen it is disadvantageous to perform the transform on the transformblock, information (i.e., transform skip information) indicating thatthe transform is not performed on the corresponding block is signaled sothat encoding is performed in pixel space without performing thetransform. Here, the transform skip information may be expressed in aflag format and may be referred to as a transform skip flag.

When the decoder decodes the transform block, the decoder first reads atransform skip flag for the block; when a value of the transform skipflag is 1, performs image decoding without performing inverse transform;and when the value of the transform skip flag is 0, performs imagedecoding by performing inverse transform. Here, the roles of 0 and 1 maybe set in reverse.

In technologies up to now, whether to skip or perform the transform hasbeen determined for each two-dimensional transform block during imageencoding. That is, it is determined whether to perform or skip thetransform on a given two-dimensional transform block withoutdistinguishing the horizontal and vertical directions with respect tothe given transform block.

However, as described above, since the characteristics of the image areoften different from each other for each of the horizontal and verticaldirections, there is a need for a technique capable of determiningwhether to skip or perform the transform independently for each of thehorizontal and vertical directions.

For example, in the case that the transform block is determined to be16×4 due to the characteristics of the image, when it is determined thatthe transform gain is sufficient in the horizontal direction having asize of 16 so that the transform is performed, but the transform gain isnot sufficient in the vertical direction having a length of 4 so that itis not advantageous to perform the transform, it may be preferable thatthe transform is performed in the horizontal direction and the transformis not performed in the vertical direction. Therefore, it is necessaryto find the conditions that the horizontal and vertical sizes of thetransform block are separately included to determining the transformskip. In addition, it is necessary to effectively encode/decode thetransform skip information independent of the horizontal and verticaldirections. In addition, it is necessary to efficiently decode thecompressed image information by performing or skipping transformindependently in vertical and horizontal directions by using the encodedindependent transform skip information in the horizontal and verticaldirections thus decoded. Here, transform skipping may have the samemeaning as transform skip.

The present invention has been devised to solve the technical problems.

A multiple transform, that is, a multiple transform selection (MTS)technique may be used for transforming an image.

The MTS technique may refer to a technique of applying varioustransforms to each of a horizontal direction and a vertical direction.

FIGS. 8 and 9 are diagrams illustrating an example of a multipletransform technique.

In FIG. 8 and FIG. 9, mts_cu_flag is information indicating whether themultiple transform technique is used. Here, mts_cu_flag may be referredto as multiple transform usage information.

For example, mts_cu_flag==0 may mean that different transform kernelsmay not be used for each of the horizontal and vertical directions, andmts_cu_flag==1 may mean that different transform kernels may be used foreach of the horizontal and vertical directions. In addition, whenmts_cu_flag==0, a transform kernel named DCT2 may be used in both thehorizontal direction and the vertical direction.

Meanwhile, the multiple transform may be used in various combinationsdepending on the image characteristics.

FIG. 8 may show an example of multiple transforms applied to an intrablock encoded by intra prediction.

In addition, FIG. 9 may show an example of multiple transforms appliedto an inter block encoded by inter prediction.

In FIG. 8 and FIG. 9, mts_index is information transmitted only whenmultiple transform is used (that is, mts_cu_flag==1) and indicatesdifferent transform kernels for each of the horizontal and verticaldirections. Here, mts_index may be referred to as multiple transformselection information.

Referring to FIGS. 8 and 9, mts_index is composed of 2 bits, and mayrepresent four combinations of transform kernels applied to a horizontaldirection and a vertical direction.

Meanwhile, the first bit and the second bit of two bits constitutingmts_index may mean a transform kernel in a horizontal direction and atransform kernel in a vertical direction, respectively.

Thus, mts_index may be represented by a combination of MTS_Hor_flag andMTS_Ver_flag. Here, MTS_Hor_flag and MTS_Ver_flag may be informationindicating a transform kernel in the horizontal direction and atransform kernel in the vertical direction, respectively.

Meanwhile, as shown in FIG. 10, when not using multiple transforms(cu_mts_flag==0), a transform skip flag(transform_skip_flag_hor/transform_skip_flag_ver) for each direction maybe signaled.

Alternatively, as shown in FIG. 11, the present invention may beimplemented so that a single transform skip flag is signaled. In thiscase, the transform skip flag for each direction may be set as follows.transform_skip_flag_hor[x0][y0][cIdx]=transform_skip_flag[x0][y0][cIdx]transform_skip_flag_ver[xz][y0][cIdx]=transform_skip_flag[x0][y0][cIdx]

Meanwhile, the encoder/decoder may signal a combination of whethertransform skip is performed and multiple transform selection.

In examples of FIGS. 8 and 9, the transform skip information isadditionally signaled when the multiple transform is not used, but anembodiment will be described hereinafter in which it is regarded thattransform in which the transform kernel is an identity matrix (IDT) isperformed at time of the transform skip.

In the present embodiment, the transform skip is substantially performedby causing transform using a transform kernel having an IDT to beselected without separately signaling transform skip information.

FIG. 12 is a diagram illustrating an embodiment in which a transformskip mode is represented as an IDT without signaling transform skipinformation.

Referring to FIG. 12, the transform skip mode may be represented by anIDT and thus represented by one transform kernel candidate of multipletransform. In detail, the transform skip information is merged intomts_Index, mts_index may indicate not only the transform kernelselection but also whether transform skip is performed or not.

FIG. 12 may be implemented as shown in FIG. 13.

Meanwhile, same_transform_hor_ver_flag, which is information indicatingwhether each of the horizontal transform and the vertical transform usesthe same transform kernel, may be used.

FIGS. 14 and 15 are diagrams illustrating various embodiments in whichinformation indicating whether a horizontal transform and a verticaltransform each use the same transform kernel is used.

Referring to FIGS. 14 and 15, when same_transform_hor_ver_flag==1, thesame transform kernel is used in the horizontal transform and verticaltransform, and thus, mts_index may indicate the transform kernel appliedto both the horizontal transform and the vertical transform. On thecontrary, when same_transform_hor_ver_flag==0, different transformkernels are used in the horizontal transform and vertical transform, andthus, mts_index may indicate transform kernels applied differently tothe horizontal transform and the vertical transform.

Here, same_transform_hor_ver_flag may be represented by theabove-described mts_cu_flag.

According to an embodiment of the present invention, thesame_transform_hor_ver_flag may be signaled as one piece of informationin combination with multiple transform selection information. That is,information indicating whether each of the horizontal transform and thevertical transform use the same transform kernel may be signaled incombination with the multiple transform selection information. Here, themultiple transform selection information may be expressed in an indexformat, and a specific index value of the multiple transform selectioninformation may indicate whether the horizontal transform and thevertical transform each use the same transform kernel.

FIG. 16 is a diagram illustrating an embodiment of using bothinformation (mts_cu_flag) indicating whether to use multiple transformand information (same_transform_hor_ver_flag) indicating whether ahorizontal transform and a vertical transform use the same transformkernel.

Referring to FIG. 16, when the multiple transform is not used (i.e.,mts_cu_flag=0), DCT2 may be used for both the horizontal transform andthe vertical transform.

In addition, when using multiple transform (mts_cu_flag==1), information(same_transform_hor_ver_flag) indicating whether the horizontaltransform and vertical transform use the same transform kernel andtransform kernel selection information (mts_index) are signaled so thatthe horizontal and vertical transforms may be determined.

Hereinafter, an order of signaling information indicating whether to usemultiple transform and transform skip information will be described.

FIGS. 17 and 18 are diagrams illustrating an embodiment in whichtransform skip information is signaled after information indicatingwhether to use multiple transform.

In FIG. 17 and FIG. 18, mts_cu_flag is information indicating whethermultiple transforms are used, and transformskip_flag_hor andtransformskip_flag_ver indicate horizontal transform skip informationand vertical transform skip information, respectively. In addition,mts_hor_index and mts_ver_index may represent horizontal transformselection information and vertical transform selection information,respectively.

In FIG. 17, mts_cu_flag may be obtained first. When the value ofmts_cu_flag is 0 (that is, when multiple transform is not used) and thecondition for the size of the current block is satisfied(TU_size_condition_satisfied), transformskip_flag_hor andtransformskip_flag_ver may be obtained.

Meanwhile, when the value of mts_cu_flag is 1 (i.e, when multipletransform is used) and the condition for the size of the current blockis satisfied (TU_size_condition_satisfied), transformskip_flag_hor andtransformskip_flag_ver are obtained, and mts_hor_index and mts_ver_indexmay be obtained on the basis of the obtained transformskip_flag_hor andtransformskip_flag_ver, respectively.

FIG. 18 is an example similar to FIG. 17, but shows an embodiment inwhich a condition for the size of the current block is separatelyapplied as a condition for the horizontal size of the current block(TU_Ver_size_condition) and a condition for the vertical size of thecurrent block (TU_Hor_size_condition). Specifically, when the conditionfor the horizontal size of the current block is satisfied(TU_Hor_size_condition_satisfied), transformskip_flag_hor may beobtained, and when the condition for the vertical size of the currentblock is satisfied (TU_Ver_size_condition_satisfied),transformskip_flag_ver may be obtained.

Here, the condition (TU_size_condition) with respect the size of thecurrent block may mean a condition of comparing the size of the currentblock with at least one of a minimum size in which transform skip ispossible or a maximum size in which transform skip is possible.

As an example, when the horizontal size of the current block and thevertical size of the current block are less than or equal to the maximumsize in which transform skip is possible, at least one of transform skipinformation and multiple transform selection information may beobtained. Here, the maximum size in which transform skip is possible maybe derived by information that is determined by the encoder and thensignaled to the decoder. Meanwhile, information on the maximum size inwhich the transform skip is possible may be signaled through a sequenceparameter set, a picture parameter set, or a slice header.

Alternatively, the maximum size in which the transform skip is possiblemay be derived on the basis of the maximum transform block size. Forexample, the maximum size may be equal to the transform block maximumsize or be ½ to the transform block maximum size.

As an example, when the horizontal size of the current block and thevertical size of the current block are less than or equal to apredetermined size, at least one of transform skip information andmultiple transform selection information may be obtained.

As an example, when the horizontal size of the current block and thevertical size of the current block are smaller than or equal to apredetermined size and smaller than or equal to a maximum size in whichtransform skip is possible, at least one of transform skip informationand multiple transform selection information may be obtained.

In the above embodiment, the predefined size may be 32.

Meanwhile, as shown in FIG. 18, a condition for comparing the horizontalsize of the current block and the vertical size of the current block maybe applied.

Here, the current block may be a transform block.

FIG. 19 is a diagram illustrating an embodiment in which informationindicating whether to use multiple transform is signaled after signalingthe transform skip information.

In FIG. 19, transformskip_flag_hor and transformskip_flag_ver may beobtained on the basis of whether the condition for the horizontal sizeof the current block is satisfied (TU_Hor_size_condition_satisfied) andwhether the condition for the vertical size of the current block issatisfied (TU_Ver_size_condition_satisfied), respectively. In addition,when at least one of the values of transformskip_flag_hor andtransformskip_flag_ver is 0 (that is, except when both a horizontaltransform skip and a vertical transform skip are applied), mts_cu_flagmay be obtained. When the value of mts_cu_flag is 0, the horizontaltransform and the vertical transform are determined as DCT2, and whenthe value of mts_cu_flag is 1, mts_hor_idx and mts_ver_idx may beobtained on the basis of transformskip_flag_hor andtransformskip_flag_ver, respectively.

FIG. 20 is a diagram illustrating an embodiment in which signaling ofmultiple transform usage information is determined on the basis ofhorizontal transform skip information and vertical transform skipinformation.

Referring to FIG. 20, when the values of transformskip_flag_hor andtransformskip_flag_ver are all 1 (that is, when both horizontaltransform skip and vertical transform skip are applied), it isdetermined that there is no possibility of MTS being to be performed, sothat mts_cu_flag may not signaled. In the opposite case, however, it isdetermined that there is a possibility of the MTS being to be performed,so that mts_cu_flag may be signaled.

Meanwhile, the multiple transform selection information may be signaledafter signaling the transform skip information.

The above-described embodiments according to the present invention areapplied to both the luma channel and the chroma channel in the same wayor independently of each other.

As an example, at least one of the transform skip and the multipletransform may be applied only to the luma channel and not to the chromachannel. As another example, transform skip may be applied to the chromaand luma channels, and multiple transform may be applied only to theluma channel.

In addition, the above-described embodiment according to the presentinvention may be independently applied to each channel (for example,RGB, YUV, YCbCr, etc.) of the image.

In addition, the method and apparatus of the present invention devisedto solve the problems described above may be equally applied to channelsof an image (e.g., RGB, YUV, YCbCr, etc.).

According to an embodiment of the present invention, information fordetermining whether to perform transform may be signaled as one piece ofinformation in combination with transform selection information. Thatis, the transform skip information may be signaled in combination withthe transform selection information. Here, the transform selectioninformation may be expressed in an index format, and a specific indexvalue of the transform selection information may indicate that thetransform is not performed.

For example, when the transform selection information is a value of 0,it may represent that the transform is not performed.

Meanwhile, the present embodiment may be applied to each of thehorizontal transform and the vertical transform, or may be applied toeach of the primary transform and the secondary transform.

FIG. 21 is a flowchart illustrating an image decoding method accordingto an embodiment of the present invention.

Referring to FIG. 21, the image decoding apparatus may obtain transformskip information of the current block from the bitstream (S2101). Here,the transform skip information may be the above-describedtransform_skip_flag.

Meanwhile, the transform skip information may include horizontaltransform skip information and vertical transform skip information.

Meanwhile, in step S2101, the transform skip information of the currentblock may be obtained when the horizontal size and the vertical size ofthe current block is less than or equal to the predetermined size. Here,the predetermined size may be 32.

In addition, in step S2101, the maximum transform skip size informationmay be obtained from the bitstream, and when the horizontal and verticalsizes of the current block are less than or equal to the maximumtransform skip size, the transform skip information of the current blockmay be obtained.

In addition, the image decoding apparatus may obtain multiple transformselection information of the current block on the basis of transformskip information from the bitstream (S2102). Here, the multipletransform selection information may be the aforementioned mts_index.Specifically, when transform skip information indicates that transformskip is not applied, multiple transform selection information of thecurrent block may be obtained.

Meanwhile, the multiple transform selection information may be used toset the horizontal transform type and the vertical transform type,respectively. Here, the transform type may be the above-describedtransform kernel.

Meanwhile, the multiple transform selection information may indicatewhether the horizontal transform type and the vertical transform typeare the same.

Meanwhile, the multiple transform selection information may be indexinformation indicating a transform type set applied to the horizontaltransform type and the vertical transform type.

In addition, the image decoding apparatus may perform inverse transformon the current block on the basis of the multiple transform selectioninformation (S2103).

FIG. 22 is a flowchart illustrating an image decoding method accordingto an embodiment of the present invention.

Referring to FIG. 22, the image decoding apparatus may obtain transformselection information of the current block from the bitstream (S2201).

In addition, the image decoding apparatus may determine a transform typeand whether to perform inverse transform on the current block on thebasis of the transform selection information (S2202). That is, thetransform selection information may include all information on whetherto perform the transform and information on transform type selection.

In addition, the image decoding apparatus may perform inverse transformon the current block according to the determination of step S2202(S2203).

Here, the inverse transform may be a secondary inverse transformperformed between inverse quantization and the first inverse transform.

FIG. 23 is a flowchart illustrating an image encoding method accordingto an embodiment of the present invention.

Referring to FIG. 23, the image encoding apparatus may determine whetherto perform transform skip of the current block (S2301).

When the transform skip is not performed on the current block(S2301-No), the image encoding apparatus may determine a horizontaltransform type and a vertical transform type of the current block(S2302).

In addition, the image encoding apparatus may perform transform on thecurrent block on the basis of the horizontal transform type and thevertical transform type (S2303).

In addition, the image encoding apparatus may encode transform skipinformation indicating whether to perform transform skip of the currentblock and multiple transform selection information indicating ahorizontal transform type and a vertical transform type of the currentblock (S2304).

Here, the multiple transform selection information may indicate whetherthe horizontal transform type and the vertical transform type are thesame.

In addition, the multiple transform selection information may be indexinformation indicating a set of transform types applied to a horizontaltransform type and a vertical transform type

Meanwhile, the image encoding apparatus may not encode transform skipinformation of the current block when the horizontal size and thevertical size of the current block are smaller than or equal to apredetermined size.

Here, the transform skip information may include horizontal transformskip information and vertical transform skip information.

Meanwhile, when the transform skip is performed on the current block(S2301—Yes), the image encoding apparatus may encode transform skipinformation indicating whether to perform the transform skip of thecurrent block (S2305).

FIG. 24 is a flowchart illustrating an image encoding method accordingto an embodiment of the present invention.

Referring to FIG. 24, the image encoding apparatus may determine whetherto perform the transform on the current block and a transform type(S2401).

In addition, the image encoding apparatus may perform transform on thecurrent block according to the determination of step S2401 (S2402).Here, the transform may be secondary transform performed between theprimary transform and the quantization.

In addition, the image encoding apparatus may encode transform selectioninformation indicating whether the transform is performed on the currentblock and a transform type (S2403).

The bitstream generated by the image encoding method according to thepresent invention may be temporarily stored in a computer-readablenon-transitory recording medium, and may be decoded by theabove-described image decoding method.

Specifically, in a non-transitory computer readable recording mediumincluding a bitstream decoded by an image decoding apparatus, thebitstream includes transform skip information of the current block andmultiple transform selection information of the current block, andtransform skip information indicates whether the inverse transform isperformed on the current block, the multiple transform selectioninformation indicates a horizontal transform type and a verticaltransform type applied to the inverse transform of the current block,and in the image decoding apparatus, the multiple transform selectioninformation is obtained on the basis of the transform skip information.

The invention claimed is:
 1. A method of decoding an image, the methodcomprising: obtaining transform skip information of a current block froma bitstream; obtaining multiple transform selection information of thecurrent block on the basis of the transform skip information from thebitstream; and performing inverse transform on the current block on thebasis of the multiple transform selection information, wherein themultiple transform selection information is used to set each of ahorizontal transform type and a vertical transform type, wherein thetransform skip information is used for both a luma component and achroma component, and wherein the multiple transform selectioninformation is used only for the luma component.
 2. The method of claim1, wherein the multiple transform selection information indicateswhether the horizontal transform type and the vertical transform typeare the same.
 3. The method of claim 1, wherein the multiple transformselection information is index information indicating a transform typeset applied to the horizontal transform type and the vertical transformtype.
 4. The method of claim 1, wherein the acquiring of the transformskip information of the current block includes: obtaining transform skipinformation of the current block when a horizontal size and a verticalsize of the current block are less than or equal to a predeterminedsize.
 5. The method of claim 4, wherein the acquiring of the transformskip information of the current block includes: obtaining maximumtransform skip size information from the bitstream; and obtainingtransform skip information of the current block when the horizontal sizeand the vertical size of the current block are less than or equal to themaximum transform skip size.
 6. The method of claim 1, wherein thetransform skip information includes horizontal transform skipinformation and vertical transform skip information.
 7. A method ofencoding an image, the method comprising: determining whether to performtransform skip of the current block; when the transform skip is notperformed on the current block, determining a horizontal transform typeand a vertical transform type of the current block; performing thetransform on the current block on the basis of the horizontal transformtype and the vertical transform type; and encoding transform skipinformation indicating whether to perform the transform skip of thecurrent block and multiple transform selection information indicatingthe horizontal transform type and the vertical transform type of thecurrent block, wherein the transform skip information is used for both aluma component and a chroma component, and wherein the multipletransform selection information is used only for the luma component. 8.The method of claim 7, wherein the multiple transform selectioninformation indicates whether the horizontal transform type and thevertical transform type are the same.
 9. The method of claim 7, whereinthe multiple transform selection information is index informationindicating a transform type set applied to the horizontal transform typeand the vertical transform type.
 10. The method of claim 7, wherein thetransform skip information of the current block is not encoded when thehorizontal size and the vertical size of the current block are less thanor equal to a predetermined size.
 11. The method of claim 7, wherein thetransform skip information includes horizontal transform skipinformation and vertical transform skip information.
 12. Anon-transitory computer readable recording medium storing a bitstreamdecoded by an image decoding apparatus, wherein the bitstream includestransform skip information of a current block and multiple transformselection information of the current block; the transform skipinformation indicates whether inverse transform is performed on thecurrent block; the multiple transform selection information indicates ahorizontal transform type and a vertical transform type applied toinverse transform of the current block; and the image decoding apparatusobtains the multiple transform selection information on the basis of thetransform skip information, wherein the transform skip information isused for both a luma component and a chroma component, and wherein themultiple transform selection information is used only for the lumacomponent.